[AST] Fix a constexpr-evaluator crash on error-dependent returnstmt.

[hokein]

Summary

When the evaluator encounters an error-dependent returnstmt, before this patch
it returned a ESR_Returned without setting the result, the callsides think this
is a successful execution, and try to access the Result which causes the crash.

The fix is to always return failed as we don't know the result of the
error-dependent return stmt.

[sammccall]

clang/lib/AST/ExprConstant.cpp

5147

this deserves a comment: // We know we returned, but we don't know what the value is.

clang/lib/AST/ExprConstant.cpp

for (const auto *D : DS->decls()) {
         return ESR_Failed;
     }
     return ESR_Succeeded;
   }
 
   case Stmt::ReturnStmtClass: {
     const Expr *RetExpr = cast<ReturnStmt>(S)->getRetValue();
     FullExpressionRAII Scope(Info);
-    if (RetExpr && RetExpr->isValueDependent())
-      return EvaluateDependentExpr(RetExpr, Info) ? ESR_Returned : ESR_Failed;
+    if (RetExpr && RetExpr->isValueDependent()) {
+      EvaluateDependentExpr(RetExpr, Info);
+      // We know we returned, but we don't know what the value is.
+      return ESR_Failed;
+    }
     if (RetExpr &&
         !(Result.Slot
               ? EvaluateInPlace(Result.Value, Info, *Result.Slot, RetExpr)
               : Evaluate(Result.Value, Info, RetExpr)))
       return ESR_Failed;
     return Scope.destroy() ? ESR_Returned : ESR_Failed;
   }
 

clang/test/SemaCXX/constexpr-function-recovery-crash.cpp

 }
 
 template<int x> constexpr int f(int y) { // expected-note {{candidate template ignored}}
   return x * y;
 }
 constexpr int test9(int x) {
   return f<1>(f<x>(1)); // expected-error {{no matching function for call to 'f'}}
 }
+
+constexpr int test10() { return undef(); } // expected-error {{use of undeclared identifier 'undef'}}
+static_assert(test10() <= 1, "should not crash"); // expected-error {{static_assert expression is not an integral constant expression}}