Implement span quoting for proc-macros

This PR implements span quoting, allowing proc-macros to produce spans
pointing *into their own crate*. This is used by the unstable
`proc_macro::quote!` macro, allowing us to get error messages like this:

```
error[E0412]: cannot find type `MissingType` in this scope
  --> $DIR/auxiliary/span-from-proc-macro.rs:37:20
   |
LL | pub fn error_from_attribute(_args: TokenStream, _input: TokenStream) -> TokenStream {
   | ----------------------------------------------------------------------------------- in this expansion of procedural macro `#[error_from_attribute]`
...
LL |             field: MissingType
   |                    ^^^^^^^^^^^ not found in this scope
   |
  ::: $DIR/span-from-proc-macro.rs:8:1
   |
LL | #[error_from_attribute]
   | ----------------------- in this macro invocation
```

Here, `MissingType` occurs inside the implementation of the proc-macro
`#[error_from_attribute]`. Previosuly, this would always result in a
span pointing at `#[error_from_attribute]`

This will make many proc-macro-related error message much more useful -
when a proc-macro generates code containing an error, users will get an
error message pointing directly at that code (within the macro
definition), instead of always getting a span pointing at the macro
invocation site.

This is implemented as follows:
* When a proc-macro crate is being *compiled*, it causes the `quote!`
  macro to get run. This saves all of the sapns in the input to `quote!`
  into the metadata of *the proc-macro-crate* (which we are currently
  compiling). The `quote!` macro then expands to a call to
  `proc_macro::Span::recover_proc_macro_span(id)`, where `id` is an
opaque identifier for the span in the crate metadata.
* When the same proc-macro crate is *run* (e.g. it is loaded from disk
  and invoked by some consumer crate), the call to
`proc_macro::Span::recover_proc_macro_span` causes us to load the span
from the proc-macro crate's metadata. The proc-macro then produces a
`TokenStream` containing a `Span` pointing into the proc-macro crate
itself.

The recursive nature of 'quote!' can be difficult to understand at
first. The file `src/test/ui/proc-macro/quote-debug.stdout` shows
the output of the `quote!` macro, which should make this eaier to
understand.

This PR also supports custom quoting spans in custom quote macros (e.g.
the `quote` crate). All span quoting goes through the
`proc_macro::quote_span` method, which can be called by a custom quote
macro to perform span quoting. An example of this usage is provided in
`src/test/ui/proc-macro/auxiliary/custom-quote.rs`

Custom quoting currently has a few limitations:

In order to quote a span, we need to generate a call to
`proc_macro::Span::recover_proc_macro_span`. However, proc-macros
support renaming the `proc_macro` crate, so we can't simply hardcode
this path. Previously, the `quote_span` method used the path
`crate::Span` - however, this only works when it is called by the
builtin `quote!` macro in the same crate. To support being called from
arbitrary crates, we need access to the name of the `proc_macro` crate
to generate a path. This PR adds an additional argument to `quote_span`
to specify the name of the `proc_macro` crate. Howver, this feels kind
of hacky, and we may want to change this before stabilizing anything
quote-related.

Additionally, using `quote_span` currently requires enabling the
`proc_macro_internals` feature. The builtin `quote!` macro
has an `#[allow_internal_unstable]` attribute, but this won't work for
custom quote implementations. This will likely require some additional
tricks to apply `allow_internal_unstable` to the span of
`proc_macro::Span::recover_proc_macro_span`.

clippy_lints/src/misc.rs

@@ -660,7 +660,7 @@ fn in_attributes_expansion(expr: &Expr<'_>) -> bool {
     use rustc_span::hygiene::MacroKind;
     if expr.span.from_expansion() {
         let data = expr.span.ctxt().outer_expn_data();
-        matches!(data.kind, ExpnKind::Macro(MacroKind::Attr, _))
+        matches!(data.kind, ExpnKind::Macro { kind: MacroKind::Attr, name: _, proc_macro: _ })
     } else {
         false
     }

clippy_lints/src/unit_types/unit_cmp.rs

@@ -8,7 +8,7 @@ use super::UNIT_CMP;
 pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>) {
     if expr.span.from_expansion() {
         if let Some(callee) = expr.span.source_callee() {
-            if let ExpnKind::Macro(MacroKind::Bang, symbol) = callee.kind {
+            if let ExpnKind::Macro { kind: MacroKind::Bang, name: symbol, proc_macro: _ } = callee.kind {
                 if let ExprKind::Binary(ref cmp, left, _) = expr.kind {
                     let op = cmp.node;
                     if op.is_comparison() && cx.typeck_results().expr_ty(left).is_unit() {

clippy_utils/src/lib.rs

@@ -947,7 +947,7 @@ pub fn is_expn_of(mut span: Span, name: &str) -> Option<Span> {
             let data = span.ctxt().outer_expn_data();
             let new_span = data.call_site;
 
-            if let ExpnKind::Macro(MacroKind::Bang, mac_name) = data.kind {
+            if let ExpnKind::Macro { kind: MacroKind::Bang, name: mac_name, proc_macro: _ } = data.kind {
                 if mac_name.as_str() == name {
                     return Some(new_span);
                 }
@@ -975,7 +975,7 @@ pub fn is_direct_expn_of(span: Span, name: &str) -> Option<Span> {
         let data = span.ctxt().outer_expn_data();
         let new_span = data.call_site;
 
-        if let ExpnKind::Macro(MacroKind::Bang, mac_name) = data.kind {
+        if let ExpnKind::Macro { kind: MacroKind::Bang, name: mac_name, proc_macro: _ } = data.kind {
             if mac_name.as_str() == name {
                 return Some(new_span);
             }