[Driver] Use llvm-config.h, not config.h to unbreak out-of-tree builds

llvm/Config/config.h has intentionally been excluded from llvm
installations (see: llvm/CMakeLists.txt). Un-break out-of-tree builds
post-r278882 by switching to llvm-config.h, which is exported.

Suggested by Will Dietz!

llvm-svn: 279035