Add LLVM 3.7 port at 3.7.0 RC1:

The LLVM Project is a collection of modular and reusable compiler and
toolchain technologies.

This port includes Clang (a C/C++/Objective-C compiler), LLD (a linker),
LLDB (a debugger), an OpenMP runtime library, and the LLVM infrastructure
these are built on.

Sponsored by: DARPA, AFRL