Documentation parsing: if typedef name is being declared

via a macro, try using declaration's starting location.
This is improvement over not having a valid location and
dropping comment altogether. // rdar://14348912


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187085 91177308-0d34-0410-b5e6-96231b3b80d8