[clang] [clang-tools-extra] [Clang][AST] Introduce `ExplicitInstantiationDecl` to preserve source info and fix diagnostic locations (PR #191658)

[Chuanqi Xu via cfe-commits]

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@@ -919,6 +919,11 @@ void ASTDeclWriter::VisitFunctionDecl(FunctionDecl *D) {
     }
   }
 
+  // Ensure associated ExplicitInstantiationDecl survives reduced BMI.
+  if (auto *EID = Record.getASTContext().getExplicitInstantiationDecl(D))
+    Writer.RelatedDeclsMap[Writer.GetDeclRef(D)].push_back(
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ChuanqiXu9 wrote:

Writer.RelatedDeclsMap not used.

https://github.com/llvm/llvm-project/pull/191658