Report an error if a #[no_mangle]/#[export_name = ...] function has the same symbol name as a built-in shim

[ghost]

Implements #1776 (comment).

The error looks like this:

error: found `malloc` symbol definition that clashes with a built-in shim
  --> tests/compile-fail/function_calls/exported_symbol_shim_clashing.rs:12:9
   |
12 |         malloc(0);
   |         ^^^^^^^^^ found `malloc` symbol definition that clashes with a built-in shim
   |
help: the `malloc` symbol is defined here

  --> tests/compile-fail/function_calls/exported_symbol_shim_clashing.rs:2:1
   |
2  | / extern "C" fn malloc(_: usize) -> *mut std::ffi::c_void {
3  | |     //~^ HELP the `malloc` symbol is defined here
4  | |     unreachable!()
5  | | }
   | |_^
   = note: inside `main` at tests/compile-fail/function_calls/exported_symbol_shim_clashing.rs:12:9

This does not implement "better error messages than we do currently for arg/ABI mismatches" in #1776 (comment) -- I failed to remove all check_arg_count() and check_abi() (they are still used in src/shims/intrinsics.rs and call_dlsym()) and they don't receive the name of the shim.

[RalfJung]

Looks great, thanks. :)
@bors r+

[bors]

📌 Commit aaaa142 has been approved by RalfJung

[bors]

⌛ Testing commit aaaa142 with merge 486b5df...

[bors]

☀️ Test successful - checks-actions
Approved by: RalfJung
Pushing 486b5df to master...