auto merge of #15511 : brson/rust/extract-rustc-back, r=alexcrichton

This was my weekend project, to start breaking up rustc. It first pulls out LLVM into `rustc_llvm`, then parts of `rustc::back` and `rustc::util` to `rustc_back`. The immediate intent is just to reduce the size of rustc, to reduce memory pressure when building rustc, but this is also a good starting point for further refactoring.

The `rustc_back` crate is definitely misnamed (`rustc::back` was never a very cohesive module anyway) - it's mostly just somewhere to stuff parts of rustc that don't have many deps. Right now it's main dep is `syntax`; it has no dep on `rustc_llvm`.

Some next steps might be to split `rustc_back` into `rustc_util` (with no `syntax` dep), and `rustc_syntax_util` (with a syntax dep); move the rest of `rustc::util` into `rustc_syntax_util`; move all of `rustc::front` to a new crate, `rustc_front`. At that point the refactoring necessary to keep extracting crates will get harder.

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