Separate out and special-case the diagnostic for 'auto' in a
conversion-type-id, in preparation for this becoming valid in c++1y mode.
No functionality change; small diagnostic improvement.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181089 91177308-0d34-0410-b5e6-96231b3b80d8