Try to fix modules build

Module builds somehow report an ambiguity between clang::Diagnostic and
clang::Tooling::Diagnostic. It seems as if one of the additional headers
brought in by the module brings the clang namespace to the toplevel. I
could not find out the reason for that, so for now I go with the simple
fix to bring the build back to green.

rdar://33321397

llvm-svn: 308071