Move VerifierSupport into namespace llvm.

It currently is in an unnamed namespace and then it shouldn't be used
from something in the header file. This actually triggers a warning with
GCC:
../include/llvm/IR/Verifier.h:39:7: warning: ‘llvm::TBAAVerifier’ has a field ‘llvm::TBAAVerifier::Diagnostic’ whose type uses the anonymous namespace [enabled by default]

llvm-svn: 289942