[clang] Reapply "[Clang][Sema] Diagnose function/variable templates that shadow their own template parameters (#78274)" (PR #79683)

[Krystian Stasiowski via cfe-commits]

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@@ -885,16 +885,19 @@ bool Sema::DiagnoseUninstantiableTemplate(SourceLocation PointOfInstantiation,
 /// that the template parameter 'PrevDecl' is being shadowed by a new
 /// declaration at location Loc. Returns true to indicate that this is
 /// an error, and false otherwise.
-void Sema::DiagnoseTemplateParameterShadow(SourceLocation Loc, Decl *PrevDecl) {
+void Sema::DiagnoseTemplateParameterShadow(SourceLocation Loc, Decl *PrevDecl,
+                                           bool IssueWarning) {
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sdkrystian wrote:

I can add javadocs for the parameters that explain the semantics of `IssueWarning` and the motivating use-case for it. Is that a good compromise?

https://github.com/llvm/llvm-project/pull/79683