Improve diagnostics when an elaborated-type-specifer containing a

Improve diagnostics when an elaborated-type-specifer containing a
nested-name-specifier (e.g., "class T::foo") fails to find a tag
member in the scope nominated by the
nested-name-specifier. Previously, we gave a bland

  error: 'Nested' does not name a tag member in the specified scope

which didn't actually say where we were looking, which was rather
horrible when the nested-name-specifier was instantiated. Now, we give
something a bit better:

  error: no class named 'Nested' in 'NoDepBase<T>'

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@100060 91177308-0d34-0410-b5e6-96231b3b80d8