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.

llvm-svn: 150487
GitOrigin-RevId: 4d9cbffbe2cfa22013e84575375d4823fc817cf2
4 files changed