Extend Sema::ClassifyName() to support C++, ironing out a few issues

Extend Sema::ClassifyName() to support C++, ironing out a few issues
in the classification of template names and using declarations. We now
properly typo-correct the leading identifiers in statements to types,
templates, values, etc. As an added bonus, this reduces the number of
lookups required for disambiguation.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@130288 91177308-0d34-0410-b5e6-96231b3b80d8