[clang] [clang][ObjC][CodeComplete] Fix crash on C-Style cast with parenthesized operand in ObjC++ (PR #180343)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> @ojhunt but the expression here seems valid? it just does not have a type yet. So the fix seems reasonable?

I thought this was incorrect handling of the injected code_completion pseudo instruction, but it's not.

I feel like the fix belongs in `CodeCompleteObjCInstanceMessage`, but this is the only code path that can hit this null - every other path has real types assigned to the subject so maybe that isn't warranted?

https://github.com/llvm/llvm-project/pull/180343