Fix crash in cleanup attr handling

ResolveSingleFunctionTemplateSpecialization() returns 0 and doesn't emit diags
unless the expression has template-ids, so we must null check the result.

Also add a better diag noting which overloads are causing the problem.

Reviewed by Aaron Ballman.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193055 91177308-0d34-0410-b5e6-96231b3b80d8