include clang's config.h unconditionally (v2)

And remove HAVE_CLANG_CONFIG_H, now that the header is generated
in the autoconf build, too.

Reverts r149571/restores r149504, now that config.h is generated
correctly by LLVM's configure in all build configurations.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@150487 91177308-0d34-0410-b5e6-96231b3b80d8