[clang] [Clang][Parser] Reject unbalanced unknown attribute arguments (PR #220259)

[Aaron Ballman via cfe-commits]

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@@ -827,7 +827,8 @@ def err_using_attribute_ns_conflict : Error<
 def err_attributes_not_allowed : Error<"an attribute list cannot appear here">;
 def err_keyword_not_allowed : Error<"%0 cannot appear here">;
 def ext_unbalanced_attribute_args : ExtWarn<
-  "attribute argument list is not a balanced token sequence">, InGroup<Pedantic>;
+  "attribute argument list is not a balanced token sequence">,
+  InGroup<DiagGroup<"unbalanced-attribute-args">>;
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AaronBallman wrote:

I think `ExtWarn` should become `Extension` and that's all that's necessary. Then it's disabled by default, but enabled via `-pedantic` but can be opted out of via `-Wno-unbalanced-attribute-args`. WDYT?

https://github.com/llvm/llvm-project/pull/220259