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

[Aaron Ballman via cfe-commits]

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@@ -1868,6 +1868,8 @@ def UnknownArgument : DiagGroup<"unknown-argument">;
 
 def UnsupportedABI : DiagGroup<"unsupported-abi">;
 
+def IgnoredAbiTagAttr : DiagGroup<"abi-tag-redeclaration">;
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AaronBallman wrote:

No need to add this here, can be done inline below.

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