Merge llvm trunk r321017 to contrib/llvm.

Merge llvm, clang, lld, lldb, compiler-rt and libc++ r305145, and update
build glue.

Update llvm to trunk r290819 and resolve conflicts.

Update llvm to release_39 branch r276489, and resolve conflicts.

Update llvm to trunk r256633.

Update llvm/clang to r241361.

Update llvm/clang to r240225.

Update Makefiles and other build glue for llvm/clang 3.7.0, as of trunk
r239412.

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.

Merge llvm 3.5.0 release from ^/vendor/llvm/dist, resolve conflicts, and
preserve our customizations, where necessary.