[CMake] Add option to link LLVM/subproject executables against LLVM libc

This patch adds in CMake option LLVM_ENABLE_LLVM_LIBC which when set to
true automatically builds LLVM libc in overlay mode and links all
generated executables against the libc overlay. This is intended to
somewhat mirror the LLVM_ENABLE_LIBCXX flag.

Differential Revision: https://reviews.llvm.org/D151013