Revert "Use function overloading instead of template specialization for diagnosis of bad template argument deductions."

This reverts commit a730f548325756d050d4caaa28fcbffdae8dfe95.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186729 91177308-0d34-0410-b5e6-96231b3b80d8