[clang-tools-extra] [clang-tidy] Fix loading clang-tidy as a Clang frontend plugin (PR #221961)

[Zeyi Xu via cfe-commits]

================
@@ -44,6 +52,8 @@ class ClangTidyPluginAction : public PluginASTAction {
     auto DiagEngine = std::make_unique<DiagnosticsEngine>(
         DiagnosticIDs::create(), *DiagOpts, DiagConsumer);
     Context->setDiagnosticsEngine(std::move(DiagOpts), DiagEngine.get());
+    DiagConsumer->BeginSourceFile(Compiler.getLangOpts(),
----------------
zeyi2 wrote:

Without this call, clang-tidy would crash with:

```
Assertion failed: (InSourceFile || Info.getLocation().isInvalid()),
function HandleDiagnostic, file ClangTidyDiagnosticConsumer.cpp, line 392.
```

https://github.com/llvm/llvm-project/pull/221961