Fix crash if initializing Clang fails. (#7248)

Flush diagnostics before destroying Clang. If we see an `inline Cpp` and
`Cpp` initialization failed, recover by skipping the inline code rather
than CHECK-failing.

toolchain/check/cpp/generate_ast.cpp

@@ -882,6 +882,13 @@ auto GenerateAst(Context& context,
   context.sem_ir().set_cpp_file(std::make_unique<SemIR::CppFile>(
       std::move(clang_instance_ptr), llvm_context));
 
+  // Register an annotation scope to flush any Clang diagnostics when we return.
+  // This ensures C++ diagnostics get flushed before `diags` is destroyed, and
+  // that diagnostics created here don't interleave with later Carbon
+  // diagnostics.
+  Diagnostics::AnnotationScope annotate_diagnostics(&context.emitter(),
+                                                    [](auto& /*builder*/) {});
+
   clang_instance.setDiagnostics(diags);
   clang_instance.setVirtualFileSystem(fs);
   clang_instance.createFileManager();
@@ -915,10 +922,6 @@ auto GenerateAst(Context& context,
     return false;
   }
 
-  // Flush any diagnostics. We know we're not part-way through emitting a
-  // diagnostic now.
-  context.emitter().Flush();
-
   return true;
 }