[clang] [clang] Use the union of abi_tags from all declarations of an inline namespace (PR #224826)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

This source code construct seems dubious; clearly nobody would ever intentionally write code like this. I think we should diagnose.

If we're worried about compatibility, we could do both: add a warning, and do the gcc-compatible thing.  But I'd like to see some evidence of real-world usage before we go that route.

https://github.com/llvm/llvm-project/pull/224826