[clang] Consider aggregate bases when checking if an InitListExpr is constant (PR #80519)

[Reid Kleckner via cfe-commits]

rnk wrote:

> Also in the issue: #80510 you pointed out a crash bug. We should have the bug covered in the test case as well.

I think that is covered by the existing test case, instead of crashing, we now reject with an error. In the future if we want to align with GCC as you suggest, we'll need to have a codegen test case for non-constant compound literals of aggregates with bases.

https://github.com/llvm/llvm-project/pull/80519