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

[Erich Keane 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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erichkeane wrote:

I concur with Aaron.  The point of this restriction is to permit the library to use attributes without the risk of someone macroing them away.  

I ALSO think that macroing away non-standards attributes should be perfectly fine, so we should only have this diagnostic for standards attributes.  I THINK we have some infrastructure already to detect those (`__has_cpp_attribute` I think also returns non-zero for non standards attributes?  But perhaps we can re-use the fact that the standards ones use a date?)?

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