[Sema] Avoid deep recursion in AnalyzeImplicitConversion

The function already exposes a work list to avoid deep recursion, this
commit starts utilizing it in a helper that could also lead to a deep
recursion.

We have observed this crash on `clang/test/C/C99/n590.c` with our
internal builds that build with aggressive optimizations and hit the
limit earlier than default release builds of Clang.

See the added test for an example with a deeper recursion that used to
crash in upstream Clang before this change with the following stack
trace:

```
  #0  llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) /usr/local/google/home/ibiryukov/code/llvm-project/llvm/lib/Support/Unix/Signals.inc:804:13
  #1  llvm::sys::RunSignalHandlers() /usr/local/google/home/ibiryukov/code/llvm-project/llvm/lib/Support/Signals.cpp:106:18
  #2  SignalHandler(int, siginfo_t*, void*) /usr/local/google/home/ibiryukov/code/llvm-project/llvm/lib/Support/Unix/Signals.inc:0:3
  llvm#3  (/lib/x86_64-linux-gnu/libc.so.6+0x3fdf0)
  llvm#4  AnalyzeImplicitConversions(clang::Sema&, clang::Expr*, clang::SourceLocation, bool) /usr/local/google/home/ibiryukov/code/llvm-project/clang/lib/Sema/SemaChecking.cpp:12772:0
  llvm#5  CheckCommaOperand /usr/local/google/home/ibiryukov/code/llvm-project/clang/lib/Sema/SemaChecking.cpp:0:3
  llvm#6  AnalyzeImplicitConversions /usr/local/google/home/ibiryukov/code/llvm-project/clang/lib/Sema/SemaChecking.cpp:12644:7
  llvm#7  AnalyzeImplicitConversions(clang::Sema&, clang::Expr*, clang::SourceLocation, bool) /usr/local/google/home/ibiryukov/code/llvm-project/clang/lib/Sema/SemaChecking.cpp:12776:5
  llvm#8  CheckCommaOperand /usr/local/google/home/ibiryukov/code/llvm-project/clang/lib/Sema/SemaChecking.cpp:0:3
  llvm#9  AnalyzeImplicitConversions /usr/local/google/home/ibiryukov/code/llvm-project/clang/lib/Sema/SemaChecking.cpp:12644:7
 llvm#10  AnalyzeImplicitConversions(clang::Sema&, clang::Expr*, clang::SourceLocation, bool) /usr/local/google/home/ibiryukov/code/llvm-project/clang/lib/Sema/SemaChecking.cpp:12776:5
 llvm#11  CheckCommaOperand /usr/local/google/home/ibiryukov/code/llvm-project/clang/lib/Sema/SemaChecking.cpp:0:3
 llvm#12  AnalyzeImplicitConversions /usr/local/google/home/ibiryukov/code/llvm-project/clang/lib/Sema/SemaChecking.cpp:12644:7
 llvm#13  AnalyzeImplicitConversions(clang::Sema&, clang::Expr*, clang::SourceLocation, bool) /usr/local/google/home/ibiryukov/code/llvm-project/clang/lib/Sema/SemaChecking.cpp:12776:5
 llvm#14  CheckCommaOperand /usr/local/google/home/ibiryukov/code/llvm-project/clang/lib/Sema/SemaChecking.cpp:0:3
 llvm#15  AnalyzeImplicitConversions /usr/local/google/home/ibiryukov/code/llvm-project/clang/lib/Sema/SemaChecking.cpp:12644:7
 llvm#16  AnalyzeImplicitConversions(clang::Sema&, clang::Expr*, clang::SourceLocation, bool) /usr/local/google/home/ibiryukov/code/llvm-project/clang/lib/Sema/SemaChecking.cpp:12776:5
 llvm#17  CheckCommaOperand /usr/local/google/home/ibiryukov/code/llvm-project/clang/lib/Sema/SemaChecking.cpp:0:3
 llvm#18  AnalyzeImplicitConversions /usr/local/google/home/ibiryukov/code/llvm-project/clang/lib/Sema/SemaChecking.cpp:12644:7
 llvm#19  AnalyzeImplicitConversions(clang::Sema&, clang::Expr*, clang::SourceLocation, bool) /usr/local/google/home/ibiryukov/code/llvm-project/clang/lib/Sema/SemaChecking.cpp:12776:5
... 700+ more stack frames.
```

Author: ilya-biryukov

clang/lib/Sema/SemaChecking.cpp

@@ -11684,15 +11684,6 @@ static void DiagnoseFloatingImpCast(Sema &S, const Expr *E, QualType T,
   }
 }
 
-static void CheckCommaOperand(Sema &S, Expr *E, QualType T, SourceLocation CC,
-                              bool ExtraCheckForImplicitConversion) {
-  E = E->IgnoreParenImpCasts();
-  AnalyzeImplicitConversions(S, E, CC);
-
-  if (ExtraCheckForImplicitConversion && E->getType() != T)
-    S.CheckImplicitConversion(E, T, CC);
-}
-
 /// Analyze the given compound assignment for the possible losing of
 /// floating-point precision.
 static void AnalyzeCompoundAssignment(Sema &S, BinaryOperator *E) {
@@ -12567,6 +12558,17 @@ struct AnalyzeImplicitConversionsWorkItem {
 };
 }
 
+static void CheckCommaOperand(
+    Sema &S, Expr *E, QualType T, SourceLocation CC,
+    bool ExtraCheckForImplicitConversion,
+    llvm::SmallVectorImpl<AnalyzeImplicitConversionsWorkItem> &WorkList) {
+  E = E->IgnoreParenImpCasts();
+  WorkList.push_back({E, CC, false});
+
+  if (ExtraCheckForImplicitConversion && E->getType() != T)
+    S.CheckImplicitConversion(E, T, CC);
+}
+
 /// Data recursive variant of AnalyzeImplicitConversions. Subexpressions
 /// that should be visited are added to WorkList.
 static void AnalyzeImplicitConversions(
@@ -12642,9 +12644,9 @@ static void AnalyzeImplicitConversions(
       /// how CheckConditionalOperand behaves; it's as-if the correct operand
       /// were directly used for the implicit conversion check.
       CheckCommaOperand(S, BO->getLHS(), T, BO->getOperatorLoc(),
-                        /*ExtraCheckForImplicitConversion=*/false);
+                        /*ExtraCheckForImplicitConversion=*/false, WorkList);
       CheckCommaOperand(S, BO->getRHS(), T, BO->getOperatorLoc(),
-                        /*ExtraCheckForImplicitConversion=*/true);
+                        /*ExtraCheckForImplicitConversion=*/true, WorkList);
       return;
     }
   }