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

[Dmitry Polukhin via cfe-commits]

dmpolukhin wrote:

Consider the standard libstdc++ idiom: c++config.h tags __cxx11 once, and `_GLIBCXX_BEGIN_NAMESPACE_CXX11` expands to a bare `namespace __cxx11 {`. Because of this, I strongly believe there should be no diagnostic when reopening a namespace without an abi_tag. The tag union behavior is crucial if a forward declaration without an abi_tag appears before the header that adds it. I agree the later cases are often not intentional, but since the tag-adding declaration is typically in a system header, a diagnostic alone would usually be suppressed and unhelpful. Moreover, mangling like the old ABI variant will result in a runtime crash in many cases.

Given this, I think the best approach is to do both: keep the GCC-compatible union behavior (which turns a runtime crash into working code) and add a warning specifically when a reopening adds a tag that the first declaration lacks.

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