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.

Author: bors

Diff for: .gitignore

@@ -86,5 +86,5 @@ src/etc/dl
 .settings/
 /build
 i686-pc-mingw32/
-src/librustc/lib/llvmdeps.rs
+src/librustc_llvm/llvmdeps.rs
 *.pot

Diff for: mk/crates.mk

@@ -53,7 +53,8 @@ TARGET_CRATES := libc std green rustuv native flate arena glob term semver \
                  uuid serialize sync getopts collections num test time rand \
                  url log regex graphviz core rlibc alloc debug rustrt \
                  unicode
-HOST_CRATES := syntax rustc rustdoc fourcc hexfloat regex_macros fmt_macros
+HOST_CRATES := syntax rustc rustdoc fourcc hexfloat regex_macros fmt_macros \
+	       rustc_llvm rustc_back
 CRATES := $(TARGET_CRATES) $(HOST_CRATES)
 TOOLS := compiletest rustdoc rustc
 
@@ -70,8 +71,10 @@ DEPS_green := std native:context_switch
 DEPS_rustuv := std native:uv native:uv_support
 DEPS_native := std
 DEPS_syntax := std term serialize log fmt_macros debug
-DEPS_rustc := syntax native:rustllvm flate arena serialize getopts \
-              time log graphviz debug
+DEPS_rustc := syntax flate arena serialize getopts \
+              time log graphviz debug rustc_llvm rustc_back
+DEPS_rustc_llvm := native:rustllvm libc std
+DEPS_rustc_back := std syntax rustc_llvm flate log libc
 DEPS_rustdoc := rustc native:hoedown serialize getopts \
                 test time debug
 DEPS_flate := std native:miniz