[clang] [Clang] Unify interface for accessing template arguments as written for class/variable template specializations (PR #81642)

[Shafik Yaghmour via cfe-commits]

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@@ -1760,6 +1760,21 @@ class BuiltinTemplateDecl : public TemplateDecl {
   BuiltinTemplateKind getBuiltinTemplateKind() const { return BTK; }
 };
 
+/// Provides information about an explicit instantiation of a variable or class
+/// template.
+struct ExplicitInstantiationInfo {
+  /// The template arguments as written..
+  const ASTTemplateArgumentListInfo *TemplateArgsAsWritten = nullptr;
+
+  /// The location of the extern keyword.
+  SourceLocation ExternLoc;
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shafik wrote:

```suggestion
  SourceLocation ExternKeywordLoc;
```

https://github.com/llvm/llvm-project/pull/81642