Avoid having "using namespace" for both "clang" and "llvm" namespaces.

This is fragile, as there are classes with the same name in both
namespaces (e.g. llvm::Module and clang::Module).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219855 91177308-0d34-0410-b5e6-96231b3b80d8