Cleanup src/librustc/back
auto merge of #7219 : sstewartgallus/rust/cleanup, r=graydon
auto merge of #7182 : Aatch/rust/trans-refactor-pt2, r=graydon

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.

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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

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EDIT: This no longer removes the llvm insn stats.