[clang/Diagnostic] Use optional to disambiguate between a StoredDiagMessage that is not set vs set as empty string

"But when would you have a completely empty diagnostic message", you ask dear reader?
That is when there is an empty "#warning" in code.

rdar://106155415

Differential Revision: https://reviews.llvm.org/D145256

(cherry picked from commit 5e03565)

Author: akyrtzi

clang/include/clang/Basic/Diagnostic.h

@@ -1564,7 +1564,7 @@ inline DiagnosticBuilder DiagnosticsEngine::Report(unsigned DiagID) {
 /// currently in-flight diagnostic.
 class Diagnostic {
   const DiagnosticsEngine *DiagObj;
-  StringRef StoredDiagMessage;
+  std::optional<StringRef> StoredDiagMessage;
 
 public:
   explicit Diagnostic(const DiagnosticsEngine *DO) : DiagObj(DO) {}

clang/lib/Basic/Diagnostic.cpp

@@ -801,8 +801,8 @@ static const char *getTokenDescForDiagnostic(tok::TokenKind Kind) {
 /// array.
 void Diagnostic::
 FormatDiagnostic(SmallVectorImpl<char> &OutStr) const {
-  if (!StoredDiagMessage.empty()) {
-    OutStr.append(StoredDiagMessage.begin(), StoredDiagMessage.end());
+  if (StoredDiagMessage.has_value()) {
+    OutStr.append(StoredDiagMessage->begin(), StoredDiagMessage->end());
     return;
   }
 

clang/unittests/Basic/DiagnosticTest.cpp

@@ -9,6 +9,7 @@
 #include "clang/Basic/Diagnostic.h"
 #include "clang/Basic/DiagnosticError.h"
 #include "clang/Basic/DiagnosticIDs.h"
+#include "clang/Basic/DiagnosticLex.h"
 #include "gtest/gtest.h"
 
 using namespace llvm;
@@ -127,4 +128,26 @@ TEST(DiagnosticTest, diagnosticError) {
   EXPECT_EQ(*Value, std::make_pair(20, 1));
   EXPECT_EQ(Value->first, 20);
 }
+
+TEST(DiagnosticTest, storedDiagEmptyWarning) {
+  DiagnosticsEngine Diags(new DiagnosticIDs(), new DiagnosticOptions);
+
+  class CaptureDiagnosticConsumer : public DiagnosticConsumer {
+  public:
+    SmallVector<StoredDiagnostic> StoredDiags;
+
+    void HandleDiagnostic(DiagnosticsEngine::Level level,
+                          const Diagnostic &Info) override {
+      StoredDiags.push_back(StoredDiagnostic(level, Info));
+    }
+  };
+
+  CaptureDiagnosticConsumer CaptureConsumer;
+  Diags.setClient(&CaptureConsumer, /*ShouldOwnClient=*/false);
+  Diags.Report(diag::pp_hash_warning) << "";
+  ASSERT_TRUE(CaptureConsumer.StoredDiags.size() == 1);
+
+  // Make sure an empty warning can round-trip with \c StoredDiagnostic.
+  Diags.Report(CaptureConsumer.StoredDiags.front());
+}
 }