Remove the last remaining llvm/Config/config.h includes

This corrects long-standing misuses of LLVM's internal config.h.

In most cases the public llvm-config.h header was intended and we can now
remove the old hacks thanks to LLVM r210144.

The config.h header is private, won't be installed and should no longer be
included by clang or other modules.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210145 91177308-0d34-0410-b5e6-96231b3b80d8