Unify build documentation and convert to reStructuredText

We now have several options that apply for both libraries and they
shouldn't be documented in multiple files. When already merging
the two Build_With_CMake.txt documents, convert them to
reStructuredText which is used for all of LLVM's documentation.

Differential Revision: https://reviews.llvm.org/D40920

llvm-svn: 321481