[test] Demonstrate bad error message

[rupprecht]

Summary

This should not be committed, but demonstrates an issue with cascading errors after D84673.

clang/unittests/Tooling/ToolingTest.cpp

InMemoryFileSystem->addFile("def/abc", 0,
                               llvm::MemoryBuffer::getMemBuffer("\n"));
   // Add a module.map file in the include directory of our header, so we trigger
   // the module.map header search logic.
   InMemoryFileSystem->addFile("def/module.map", 0,
                               llvm::MemoryBuffer::getMemBuffer("\n"));
   EXPECT_TRUE(Invocation.run());
 }
 
+class LogDiagnosticConsumer : public DiagnosticConsumer {
+public:
+  LogDiagnosticConsumer() = default;
+
+  void HandleDiagnostic(clang::DiagnosticsEngine::Level,
+                        const clang::Diagnostic &info) override {
+    llvm::SmallString<40> diagnostic_string;
+
+    info.FormatDiagnostic(diagnostic_string);
+    Diagnostics += diagnostic_string.str();
+
+    // The next expression triggers:
+    //   Assertion `SourceMgr && "SourceManager not set!"' failed.
+    // When commented out, will log the diagnostic as normal, which is an
+    // invalid flag:
+    //   "invalid integral value '-1' in '-ferror-limit -1'"
+    info.getSourceManager();
+  }
+  std::string Diagnostics;
+};
+
+TEST(ToolInvocation, DiagCallback) {
+  llvm::IntrusiveRefCntPtr<llvm::vfs::OverlayFileSystem> OverlayFileSystem(
+      new llvm::vfs::OverlayFileSystem(llvm::vfs::getRealFileSystem()));
+  llvm::IntrusiveRefCntPtr<llvm::vfs::InMemoryFileSystem> InMemoryFileSystem(
+      new llvm::vfs::InMemoryFileSystem);
+  OverlayFileSystem->pushOverlay(InMemoryFileSystem);
+  llvm::IntrusiveRefCntPtr<FileManager> Files(
+      new FileManager(FileSystemOptions(), OverlayFileSystem));
+  std::vector<std::string> Args;
+  Args.push_back("tool-executable");
+  // Note: intentional error; user probably meant -ferror-limit=0.
+  Args.push_back("-ferror-limit=-1");
+  Args.push_back("-fsyntax-only");
+  Args.push_back("test.cpp");
+  clang::tooling::ToolInvocation Invocation(
+      Args, std::make_unique<SyntaxOnlyAction>(), Files.get());
+  InMemoryFileSystem->addFile(
+      "test.cpp", 0, llvm::MemoryBuffer::getMemBuffer("int main(){}\n"));
+
+  LogDiagnosticConsumer LDC;
+  Invocation.setDiagnosticConsumer(&LDC);
+
+  EXPECT_TRUE(Invocation.run());
+
+  EXPECT_EQ("", LDC.Diagnostics);
+}
+
 struct VerifyEndCallback : public SourceFileCallbacks {
   VerifyEndCallback() : BeginCalled(0), EndCalled(0), Matched(false) {}
   bool handleBeginSource(CompilerInstance &CI) override {
     ++BeginCalled;
     return true;
   }
   void handleEndSource() override { ++EndCalled; }
   std::unique_ptr<ASTConsumer> newASTConsumer() {