Upgrade our copies of clang, llvm, lldb, compiler-rt and libc++ to 3.9.0
release, and add lld 3.9.0. Also completely revamp the build system for
clang, llvm, lldb and their related tools.

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, clang and lld are available here:
http://llvm.org/releases/3.9.0/docs/ReleaseNotes.html
http://llvm.org/releases/3.9.0/tools/clang/docs/ReleaseNotes.html
http://llvm.org/releases/3.9.0/tools/lld/docs/ReleaseNotes.html

Thanks to Ed Maste, Bryan Drewery, Andrew Turner, Antoine Brodin and Jan
Beich for their help.

Relnotes: yes
MFC after: 1 month