[clangd] Respect WarningsAsErrors configuration for clang-tidy

[nridge]

Summary

This completes the fix for https://bugs.llvm.org/show_bug.cgi?id=41218.

[ilya-biryukov]

clang-tools-extra/clangd/ClangdUnit.cpp

315

Why can't we call CTContext->treatAsError() here?

[nridge]

Address review comment

[nridge]

clang-tools-extra/clangd/ClangdUnit.cpp

315

Good point, that makes this much easier. Thanks!

[nridge]

Update as per changes to dependent patch D60953

[nridge]

Changing reviewer to @sammccall as the updates to the patch are closely related to the discussion in D60953.

Also added a couple of test cases.

[sammccall]

LG, thanks for the changes!

clang-tools-extra/clangd/ClangdUnit.cpp

suppressDiagnostics([this](DiagnosticsEngine::Level DiagLevel,
       return false;
     });
   }
 
   void setClangTidyContext(tidy::ClangTidyContext *CTContext) {
     this->CTContext = CTContext;
   }
 
+  void HandleDiagnostic(DiagnosticsEngine::Level DiagLevel,
+                        const clang::Diagnostic &Info) override {
+    DiagnosticsEngine::Level Level = DiagLevel;
+
+    // Check for warning-as-error.
+    if (CTContext && DiagLevel == DiagnosticsEngine::Warning) {
+      std::string CheckName = CTContext->getCheckName(Info.getID());
+      if (!CheckName.empty() && CTContext->treatAsError(CheckName)) {
+        Level = DiagnosticsEngine::Error;
+      }
+    }
+
+    StoreDiags::HandleDiagnostic(Level, Info);
+  }
+
 private:
   tidy::ClangTidyContext *CTContext = nullptr;
 };
 
 } // namespace
 
 void dumpAST(ParsedAST &AST, llvm::raw_ostream &OS) {
   AST.getASTContext().getTranslationUnitDecl()->dump(OS, true);