[clang] [Clang] restrict use of attribute names reserved by the C++ standard (PR #106036)

[Aaron Ballman via cfe-commits]

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@@ -177,6 +177,26 @@ static bool isLanguageDefinedBuiltin(const SourceManager &SourceMgr,
   return false;
 }
 
+static bool isReservedAttrName(Preprocessor &PP, IdentifierInfo *II) {
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AaronBallman wrote:

Because we don't support any non-standard attributes without a vendor namespace (e.g., we don't support `[[foo]]` as an extension, it needs to have some sort of prefix like `[[gnu::foo]]` or `[[clang::foo]]`), I was thinking we could rely on that.

However, it might also make sense to just add a bit to the `Spelling` in `Attr.td` to say "this is a standard spelling for the attribute" (which side-steps things like `nodiscard` vs `warn_unused_result`)?

https://github.com/llvm/llvm-project/pull/106036