Don't emit undefined-internal warnings for CXXDeductionGuideDecls.

Patch by ~paul (cynecx on phabricator)! Some test massaging by me.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@309975 91177308-0d34-0410-b5e6-96231b3b80d8

Author: zygoloid