accurate by asking the parser whether there was an ambiguity rather than trying
to reverse-engineer it from the DeclSpec. Make the with-parameters case have
better diagnostics by using semantic information to drive the warning,
improving the diagnostics and adding a fixit.

Patch by Nikola Smiljanic. Some minor changes by me to suppress diagnostics for
declarations of the form 'T (*x)(...)', which seem to have a very high false
positive rate, and to reduce indentation in 'warnAboutAmbiguousFunction'.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@160998 91177308-0d34-0410-b5e6-96231b3b80d8