Fix errored return value in CheckFunctionReturnType and add a fixit hint

As discovered by ChenWJ and listed on cfe-dev, the error for Objective C
return type ended up being wrong. This fixes that. Additionally, as a
"while we're there", the other usages of this error and the usage of the
FP above both use a FixItHint, so I'll add it here.

Differential Revision: https://reviews.llvm.org/D32759