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

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

Propagating the abi tag from the first namespace declaration to later declarations is fine; the problem is propagating tags backwards.

In terms of the implementation, the way this patch is written, it has the side-effect that the mangling of a symbol depends on when we decide to mangle it.  We typically don't wait for the end of the translation unit before we start mangling values.

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