[NFC] add DiagnosticsTestHelper decl (#112820)

This is one of the many PRs to fix errors with LLVM_ENABLE_WERROR=on.
Built by GCC 11.

Fix warning

llvm-project/clang/unittests/Basic/DiagnosticTest.cpp:19:6: error: ‘void
clang::DiagnosticsTestHelper(clang::DiagnosticsEngine&)’ has not been
declared within ‘clang’ [-Werror]
   19 | void clang::DiagnosticsTestHelper(DiagnosticsEngine &diag) {
      |      ^~~~~
In file included from
llvm-project/clang/unittests/Basic/DiagnosticTest.cpp:9:
llvm-project/clang/include/clang/Basic/Diagnostic.h:567:15: note: only
here as a ‘friend’
  567 |   friend void DiagnosticsTestHelper(DiagnosticsEngine &);

Author: jsji

clang/unittests/Basic/DiagnosticTest.cpp

@@ -16,6 +16,11 @@
 using namespace llvm;
 using namespace clang;
 
+// Declare DiagnosticsTestHelper to avoid GCC warning
+namespace clang {
+void DiagnosticsTestHelper(DiagnosticsEngine &diag);
+}
+
 void clang::DiagnosticsTestHelper(DiagnosticsEngine &diag) {
   EXPECT_FALSE(diag.DiagStates.empty());
   EXPECT_TRUE(diag.DiagStatesByLoc.empty());