[Clang] Modify sanity check assert in AggExprEmitter::VisitInitListExpr to...

[Clang] Modify sanity check assert in AggExprEmitter::VisitInitListExpr to cover anonymous struct in a union GNU extension

AggExprEmitter::VisitInitListExpr sanity checks that an empty union is really
empty and not a semantic analysis failure. The assert is missing that we allow
anonymous structs as a GNU extension. I have updated the assert to take that into account.

This fixes: https://github.com/llvm/llvm-project/issues/58800

Differential Revision: https://reviews.llvm.org/D139261