[clang] [clang-tools-extra] Fix OOM in FormatDiagnostic (PR #108187)

[Aaron Ballman via cfe-commits]

================
@@ -538,24 +511,51 @@ bool DiagnosticsEngine::EmitCurrentDiagnostic(bool Force) {
     Emitted = (DiagLevel != DiagnosticIDs::Ignored);
     if (Emitted) {
       // Emit the diagnostic regardless of suppression level.
-      Diags->EmitDiag(*this, DiagLevel);
+      Diags->EmitDiag(*this, DB, DiagLevel);
     }
   } else {
     // Process the diagnostic, sending the accumulated information to the
     // DiagnosticConsumer.
-    Emitted = ProcessDiag();
+    Emitted = ProcessDiag(DB);
   }
 
-  // Clear out the current diagnostic object.
-  Clear();
+  return Emitted;
+}
 
-  // If there was a delayed diagnostic, emit it now.
-  if (!Force && DelayedDiagID)
-    ReportDelayed();
+DiagnosticBuilder::DiagnosticBuilder(DiagnosticsEngine *diagObj,
+                                     SourceLocation CurDiagLoc,
+                                     unsigned CurDiagID)
+    : StreamingDiagnostic(diagObj->DiagAllocator), DiagObj(diagObj),
+      CurDiagLoc(CurDiagLoc), CurDiagID(CurDiagID), IsActive(true) {
+  assert(diagObj && "DiagnosticBuilder requires a valid DiagnosticsEngine!");
+}
 
-  return Emitted;
+DiagnosticBuilder::DiagnosticBuilder(const DiagnosticBuilder &D)
+    : StreamingDiagnostic() {
+  CurDiagLoc = D.CurDiagLoc;
+  CurDiagID = D.CurDiagID;
+  FlagValue = D.FlagValue;
+  DiagObj = D.DiagObj;
+  DiagStorage = D.DiagStorage;
+  D.DiagStorage = nullptr;
----------------
AaronBallman wrote:

Hmmm the downside to this is that someone may be expecting a copy operation and be caught off-guard by the copy constructor mutating the original object. Perhaps a bad idea, but I wonder if maybe we should have a move constructor that needs to be used if there's allocations by `DiagStorageAllocator` and the copy constructor can be used otherwise?

https://github.com/llvm/llvm-project/pull/108187