[analyzer] RetainCountChecker: don't assume all functions have names.

Fixes a hard-to-reach crash when calling a non-member overloaded operator
with arguments that may be callbacks.

Future-proofing: don't make the same assumption in MallocSizeofChecker.
Aside from possibly respecting attributes in the future, it might be
possible to call 'malloc' through a function pointer.

I audited all other uses of FunctionDecl::getIdentifier() in the analyzer;
they all now correctly test to see if the identifier is present before
using it.

llvm-svn: 163012