[clang] [clang] Diagnose when spelled 'abi_tag' on namespace doesn't match applied one. (PR #221039)

[Dmitry Polukhin via cfe-commits]

dmpolukhin wrote:

@erichkeane the problem it diagnoses: for an inline namespace, Clang takes ABI tags only from the *first* declaration and silently ignores tags added on a reopening (GCC unions them). The realistic case is a user forward declaration that comes before libstdc++:

```cpp
namespace std::filesystem { inline namespace __cxx11 { class path; } }  // user header, no tag
#include <filesystem>   // libstdc++ declares: inline namespace __cxx11 __attribute__((__abi_tag__("cxx11")))
```

Clang then drops `cxx11` for the whole TU, so e.g. `current_path()` binds to libstdc++'s old-ABI symbol: it links and crashes at run time. That's why the warning is anchored at the first declaration: it's the line the user has to fix, and the tag-adding declaration is usually in a system header where a warning would be suppressed.

Agreed the text should say that directly. How about stating the rule and naming the ignored tags (prototyped on the current head):

```
fwd.cpp:2:46: warning: ABI tag "cxx11" on a later declaration of inline namespace '__cxx11' is ignored; this first declaration has no 'abi_tag' [-Wabi-tag-redeclaration]
sys/bits/fs_fwd.h:3:41: note: later declaration of '__cxx11' is here

tags.cpp:1:36: warning: ABI tags "X", "Y" on a later declaration of inline namespace 'v1' are ignored; only 'abi_tag("A", "B")' from this first declaration is used [-Wabi-tag-redeclaration]
tags.cpp:2:36: note: later declaration of 'v1' is here
```

```tablegen
def warn_abi_tag_ignored : Warning<
  "ABI %plural{1:tag|:tags}0 \"%1\" on a later declaration of inline namespace "
  "%2 %plural{1:is|:are}0 ignored; %select{this first declaration has no "
  "'abi_tag'|only 'abi_tag(\"%4\")' from this first declaration is used}3">,
  InGroup<DiagGroup<"abi-tag-redeclaration">>;
def note_abi_tag_ignored_later : Note<"later declaration of %0 is here">;
```


https://github.com/llvm/llvm-project/pull/221039