Merge llvm trunk r321017 to contrib/llvm.

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

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.

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