Make the mangled name collision diagnostic a bit more useful by listing the mangling.

Make the mangled name collision diagnostic a bit more useful by listing the mangling.

This helps especially when the collision is for a template specialization,
where the template arguments are not available from anywhere else in the
diagnostic, and are likely relevant to the problem.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@333489 91177308-0d34-0410-b5e6-96231b3b80d8