Fix for PR16570: when comparing two function pointers, discard qualifiers when
comparing non-reference function parameters.  The qualifiers don't matter for
comparisons.

This is a re-commit of r187769, which was accidentially reverted in r187770,
with a simplification at the suggestion of Eli Friedman.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188112 91177308-0d34-0410-b5e6-96231b3b80d8