This also provides isConst/Volatile/Restrict on FunctionTypes to coalesce
the implementation with other callers (& update those other callers).

Patch contributed by Sam Panzer (panzer@google.com).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@161647 91177308-0d34-0410-b5e6-96231b3b80d8