Merge llvm, clang, lld, lldb, compiler-rt and libc++ r306325, and update
build glue.

Merge llvm trunk r300422 and resolve conflicts.

Update llvm to release_39 branch r276489, and resolve conflicts.

Update llvm to trunk r256945.

Update llvm to trunk r256633.

Merge llvm trunk r238337 from ^/vendor/llvm/dist, resolve conflicts, and
preserve our customizations, where necessary.

Merge llvm 3.6.0rc1 from ^/vendor/llvm/dist, merge clang 3.6.0rc1 from
^/vendor/clang/dist, resolve conflicts, and cleanup patches.