Fix the behavior of -Wignored-qualifiers on return type qualifiers in

Fix the behavior of -Wignored-qualifiers on return type qualifiers in
several ways. We now warn for more of the return types, and correctly
locate the ignored ones. Also adds fix-it hints to remove the ignored
qualifiers. Fixes much of PR9058, although not all of it.

Patch by Hans Wennborg, a couple of minor style tweaks from me.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@126321 91177308-0d34-0410-b5e6-96231b3b80d8