[CodeGen] Handle __func__ inside __finally

When we enter a __finally block, the CGF's CurCodeDecl will be null
(because CodeGenFunction::StartFunction is given an empty GlobalDecl for
a __finally block), and so the dyn_cast here will result in an assertion
failure. Change it to dyn_cast_or_null to handle this case.

Differential Revision: https://reviews.llvm.org/D45523

llvm-svn: 329836