static analyzer: handle casts of a function to a function pointer with

static analyzer: handle casts of a function to a function pointer with
a different return type.  While we don't emit any errors (yet), at
least we avoid cases where we might crash because of an assertion
failure later on (when the return type differs from what is expected).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@95268 91177308-0d34-0410-b5e6-96231b3b80d8