Replace dynamic_cast with virtual call (microsoft#6515)

Make TextDiagnosticPrinter::setPrefix a virtual function in base class
DiagnosticConsumer. This allows us to avoid using dynamic_cast in
BackendConsumer::DxilDiagHandler, required for codebases that do not
enable RTTI. This is also the only place in the codebase that uses RTTI
(AFAICT).

Author: amaiorano

tools/clang/include/clang/Basic/Diagnostic.h

@@ -1395,6 +1395,8 @@ class DiagnosticConsumer {
   /// warnings and errors.
   virtual void HandleDiagnostic(DiagnosticsEngine::Level DiagLevel,
                                 const Diagnostic &Info);
+
+  virtual void setPrefix(std::string Value) {} // HLSL Change
 };
 
 /// \brief A diagnostic client that ignores all diagnostics.

tools/clang/include/clang/Frontend/TextDiagnosticPrinter.h

@@ -45,7 +45,8 @@ class TextDiagnosticPrinter : public DiagnosticConsumer {
   /// setPrefix - Set the diagnostic printer prefix string, which will be
   /// printed at the start of any diagnostics. If empty, no prefix string is
   /// used.
-  void setPrefix(std::string Value) { Prefix = Value; }
+  // HLSL Change: add override
+  void setPrefix(std::string Value) override { Prefix = Value; }
 
   void BeginSourceFile(const LangOptions &LO, const Preprocessor *PP) override;
   void EndSourceFile() override;

tools/clang/lib/CodeGen/CodeGenAction.cpp

@@ -557,7 +557,7 @@ BackendConsumer::DxilDiagHandler(const llvm::DiagnosticInfoDxil &D) {
 
   // If no location information is available, add function name
   if (Loc.isInvalid()) {
-    auto *DiagClient = dynamic_cast<TextDiagnosticPrinter*>(Diags.getClient());
+    auto *DiagClient = Diags.getClient();
     auto *func = D.getFunction();
     if (DiagClient && func)
       DiagClient->setPrefix("Function: " + func->getName().str());