[llvm][DiagnosticInfo] handle function pointer casts

As pointed out by @arsenm in https://reviews.llvm.org/D141451#4045099,
we don't handle ConstantExpressions for dontcall-{warn|error} IR Fn
Attrs.

Use CallBase::getCalledOperand() and Value::stripPointerCasts() should
the call to CallBase::getCalledFunction return nullptr.

I don't know how to express the IR test case in C, otherwise I'd add a
clang test, too.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D142058