r238389 - [omp] Re-work Clang's handling of -fopenmp and undo r237769.

[Andrey Bokhanko]

Chandler,

> +set(CLANG_DEFAULT_OPENMP_RUNTIME "libgomp" CACHE STRING
> +  "Default OpenMP runtime used by -fopenmp.")

This effectively invalidates instructions recently published on LLVM
blog (http://blog.llvm.org/2015/05/openmp-support_22.html) and picked
up in many places.

> To re-enable LLVM's OpenMP runtime (which I think should wait until the
> normal getting started instructions are a reasonable way for falks to
> check out, build, and install Clang with the runtime)

Why this readme: http://openmp.llvm.org/README.txt (easily accessible
from openmp.llvm.org) is not sufficient?

Also, have you reviewed your patch with OpenMP in Clang code owner
(Alexey Bataev)? Or at all?...

Yours,
Andrey