[clang] [Clang][AST] Fix ExplicitInstantiationDecl accessors for variable templates with tag types (PR #197856)

[Yanzuo Liu via cfe-commits]

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@@ -3449,9 +3449,19 @@ class ExplicitInstantiationDecl final
     return hasTrailingQualifier() ? 1 : 0;
   }
 
-  /// Raw access to the internal TypeSourceInfo.  For class templates this is
-  /// a TemplateSpecializationTypeLoc; for nested classes a TagTypeLoc.
-  /// Public getTypeAsWritten() returns null for those cases.
+  /// For class templates / nested classes, returns the TypeLoc encoding the
+  /// entity (TemplateSpecializationTypeLoc or TagTypeLoc).  For function /
+  /// variable templates -- where TypeSourceInfo holds the declared type
+  /// rather than the entity -- returns std::nullopt.
+  std::optional<TypeLoc> getClassTypeLoc() const {
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zwuis wrote:

Can we return default-constructed `TypeLoc` instead of `std::nullopt`?

https://github.com/llvm/llvm-project/pull/197856