SILGen: Add missing FormalEvaluationScope when evaluating consumed noncopyable switch subjects.

After #83959, we expect noncopyable OpenExistential evaluations to have an outer
FormalEvaluationScope to forward to. However, there was no such
FormalEvaluationScope while emitting a consuming switch subject, causing an assertion
failure to trip. Add a FormalEvaluationScope tightly scoped around the evaluation in
this case, fixing rdar://160080337.

Author: jckarter

lib/SILGen/SILGenPattern.cpp

@@ -3500,7 +3500,7 @@ void SILGenFunction::emitSwitchStmt(SwitchStmt *S) {
   //   exits out of the switch.
   //
   // When we break out of a case block, we take the subject's remnants with us
-  // in the former case, but not the latter.q
+  // in the former case, but not the latter.
   CleanupsDepth subjectDepth = Cleanups.getCleanupsDepth();
   LexicalScope switchScope(*this, CleanupLocation(S));
   std::optional<FormalEvaluationScope> switchFormalAccess;
@@ -3563,6 +3563,10 @@ void SILGenFunction::emitSwitchStmt(SwitchStmt *S) {
   }
   case ValueOwnership::Owned: {
     // A consuming pattern match. Emit as a +1 rvalue.
+    // Create a tight evaluation scope for temporary borrows emitted during the
+    // evaluation.
+    FormalEvaluationScope limitedScope(*this);
+
     subjectMV = emitRValueAsSingleValue(S->getSubjectExpr());
     break;
   }

test/SILGen/switch_noncopyable_consumed_subject_with_open_existential.swift

@@ -0,0 +1,17 @@
+// RUN: %target-swift-emit-silgen -verify %s
+
+func fooify(p: any P) {
+  switch p.foo() {
+  case .success:
+    break
+  }
+}
+
+enum Resoult: ~Copyable {
+  case success
+}
+
+protocol P {
+  func foo() -> Resoult
+}
+