include clang's config.h unconditionally

And remove HAVE_CLANG_CONFIG_H, now that the header is generated
in the autoconf build, too. (clang r149497 / llvm r149498)

Also include the config.h header after all other headers, per
the LLVM coding standards.

It also turns out WindowsToolChain.cpp wasn't using the config
header at all, so that include's just deleted now.

llvm-svn: 149504