LLVM_ENABLE_MODULES is sensitive of -D. Move them into config.h.

FIXME: It'd be better that they are #cmakedefine01 rather than #cmakedefine.
(#if FOO rather than #if defined(FOO))
Then we can find missing #include "clang/Config/config.h" in the future.

Differential Revision: https://reviews.llvm.org/D35527

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@308277 91177308-0d34-0410-b5e6-96231b3b80d8