[AST] Fix certain consteval assignment and comma operator issues with fixed-point types.

[ebevhan]

Summary

Assignment and comma operators for fixed-point types were being constevaled as other
binary operators, but they need special treatment.

[ebevhan]

Rebased.

[ebevhan]

Rebased.

clang/lib/AST/ExprConstant.cpp

bool FixedPointExprEvaluator::VisitCastExpr(const CastExpr *E) {
   case CK_LValueToRValue:
     return ExprEvaluatorBaseTy::VisitCastExpr(E);
   default:
     return Error(E);
   }
 }
 
 bool FixedPointExprEvaluator::VisitBinaryOperator(const BinaryOperator *E) {
+  if (E->isPtrMemOp() || E->isAssignmentOp() || E->getOpcode() == BO_Comma)
+    return ExprEvaluatorBaseTy::VisitBinaryOperator(E);
+
   const Expr *LHS = E->getLHS();
   const Expr *RHS = E->getRHS();
   FixedPointSemantics ResultFXSema =
       Info.Ctx.getFixedPointSemantics(E->getType());
 
   APFixedPoint LHSFX(Info.Ctx.getFixedPointSemantics(LHS->getType()));
   if (!EvaluateFixedPointOrInteger(LHS, LHSFX, Info))
     return false;

clang/test/Frontend/fixed_point_crash.c

+// RUN: %clang_cc1 -verify -ffixed-point %s
+
+union a {
+  _Accum x;
+  int i;
+};
+
+int fn1() {
+  union a m;
+  m.x = 5.6k;
+  return m.i;
+}
+
+int fn2() {
+  union a m;
+  m.x = 7, 5.6k; // expected-warning {{expression result unused}}
+  return m.x, m.i; // expected-warning {{expression result unused}}
+}
+
+_Accum acc = (0.5r, 6.9k); // expected-warning {{expression result unused}}