[clang] [Clang][Sema] Implement approved resolution for CWG2858 (PR #88042)

[Krystian Stasiowski via cfe-commits]

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@@ -58,3 +58,24 @@ void B<int>::g() requires true;
 #endif
 
 } // namespace dr2847
+
+namespace dr2858 { // dr2858: 19
+
+#if __cplusplus > 202302L
+
+template<typename... Ts>
+struct A {
+  // FIXME: The nested-name-specifier in the following friend declarations are declarative,
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sdkrystian wrote:

Not calling `diagnoseQualifiedDeclaration` for friend declarations is unrelated to this DR. Besides that, the only reason we can't write a test to produce this diagnostic is because we compute the `DeclContext` of the _nested-name-specifier_ _before_ `diagnoseQualifiedDeclaration` is called. 

https://github.com/llvm/llvm-project/pull/88042