Upgrade our copies of clang, llvm, lldb, compiler-rt and libc++ to 3.7.0
release.

Please note that from 3.5.0 onwards, clang, llvm and lldb require C++11
support to build; see UPDATING for more information.

Release notes for llvm and clang can be found here:
http://llvm.org/releases/3.7.0/docs/ReleaseNotes.html
http://llvm.org/releases/3.7.0/tools/clang/docs/ReleaseNotes.html

Thanks to Ed Maste, Andrew Turner and Antoine Brodin for their help.

Exp-run: antoine
Relnotes: yes