[clang] [clang] Fix a crash issue that caused by handling of fields with initializers in nested anonymous unions (PR #113049)

[via cfe-commits]

yronglin wrote:

> LGTM
> 
> I'm a little concerned this is going to lead to other crashes due to exposing more codepaths to "invalid" decls. But it seems like it's doing the right thing in the given cases.

Yeah, if the PR breaks clang, I'll revert it quickly.

https://github.com/llvm/llvm-project/pull/113049