Trans Refactor: Episode 2

[Aatch]

This is another big refactoring of trans though this is unlikely to have much of an
impact on code size or speed.

The major change here is the implementation of a Type struct which is the new
home for all your LLVM TypeRef needs. It's a simple wrapper struct, with static
methods for constructing types, then regular methods for
manipulating/interrogating them. The purpose of this is mostly to make the code
surrounding them somewhat more ideomatic. A line like: T_ptr(T_ptr(T_i8())) is
now Type::i8().ptr_to().ptr_to(),which is much more like regular Rust code.

There are a variety of smaller changes here and there:

• Remove address spaces. At least it doesn't generate them, I haven't spent much
  time looking for related code.
• Use a macro for declaring the LLVM intrinsics, makes it look much nicer.
• Make the type for a string slice actually create a named str_slice type in LLVM,
  this makes reading the appropriate code much easier.
• Change the way struct and enum type names are generated. This just means
  that a struct like struct Foo { a: int } now produces the IR
  %struct.Foo = type { i64 }, which is much easier to read. Similarly, other structs
  are a bit tighter to make it easier to read.

---

This PR did get away from me a little, as I occasionally got distracted or as I fixed
up problems with unrelated code that were stopping me from continuing. One major
thing is that this PR contains the work from #7168, since that would have conflicted
with this and it was broken anyway. Sorry for bundling it like this.

Fixes #3670 and #7063

---

EDIT: This no longer removes the llvm insn stats.

[graydon]

count-llvm-insns has helped us find several major codegen problems, I'd prefer reworking it somehow, not removing it.

[graydon]

Possible: sink it into the block wrapper struct, such that it reports a hierarchy of blocks, not translation fn calls? Alternative: tls?

[Aatch]

@graydon I removed the offending commit (I did see this coming, so deliberately made it easier for me to undo) and re-based. I'm just checking it now.

I'll re-implement the counter using task-local storage, since it makes a bit more sense anyway.

[Aatch]

@graydon I fixed the issue with the error messages and took the opportunity to clean up the ppaux.rs code a little and make sure that the parameterized lifetimes print properly. Originally I changed it to print &'a Foo, but it seems that isn't actually correct for that case, since it's actually a parameter on Foo not a region pointer. So it now correctly prints Foo<'a> and the tests reflect that.

Because of that, this code:

fn foo<'a, 'b, T>(a: Foo<'a,T>) -> Foo<'b,T> {
    a
}

would produce an error something like: expected Foo<'b,'a> found Foo<'a,'a> which is confusing, so this also changes the way type parameters are printed by starting at 'T' and wrapping around to 'A' if the character goes above 'Z'. I made the assumption that if you have > 26 type parameters then you have bigger problems than seeing 'T' twice.

In short, printing a type like Foo<'r,T,R,I> produces Foo<'r,T,U,V> instead of Foo<'r,'a,'b,'c>.

I also added the commit I mentioned to make count-llvm-insn stuff use task-local storage instead of being directly attached to the crate context, though the stats themselves are still on the crate context.

[emberian]