Improve diagnostics for the "type qualifier on return type has no

effect warning" by printing the qualifiers we saw and correctly
pluralizing the message, e.g.,

test/SemaCXX/conditional-expr.cpp:295:3: warning: 'const volatile' type
      qualifiers on return type have no effect
  const volatile Enum g2() {
  ^~~~~ ~~~~~~~~

llvm-svn: 108236