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

[Aaron Ballman via cfe-commits]

================
@@ -6486,6 +6486,9 @@ def err_redefinition_extern_inline : Error<
 def warn_attr_abi_tag_namespace : Warning<
   "'abi_tag' attribute on %select{non-inline|anonymous}0 namespace ignored">,
   InGroup<IgnoredAttributes>;
+def warn_abi_tag_ignored_different : Warning<
+  "%select{no 'abi_tag'|'abi_tag' %1}0 prevents applying 'abi_tag' %2 later">,
----------------
AaronBallman wrote:

```suggestion
  "%select{no 'abi_tag'|'abi_tag(\"%1\")'}0 prevents applying 'abi_tag(\"%2\")' later">,
```

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