Extend the member function discovery APIs to also support Objective-C as well as C++

For the Objective-C case, we do not have a "function type" notion, so we actually end up wrapping the clang ObjCMethodDecl in the Impl object, and ask function-y questions of it
In general, you can always ask for return type, number of arguments, and type of each argument using the TypeMemberFunction layer - but in the C++ case, you can also acquire a Type object for the function itself, which instead you can't do in the Objective-C case

llvm-svn: 218132