RequirementMachine: Fix reference invalidation bug

After forming a reference to an entry in a DenseMap, we have to be
careful not touch the reference after calling any code which might
re-entrantly invalidate the reference by mutating the DenseMap.

Fixes rdar://problem/83891298.

Author: slavapestov

lib/AST/RequirementMachine/RewriteContext.cpp

@@ -367,10 +367,13 @@ RequirementMachine *RewriteContext::getRequirementMachine(
   // Store this requirement machine before adding the signature,
   // to catch re-entrant construction via initWithGenericSignature()
   // below.
-  machine = new rewriting::RequirementMachine(*this);
-  machine->initWithGenericSignature(sig);
+  auto *newMachine = new rewriting::RequirementMachine(*this);
+  machine = newMachine;
 
-  return machine;
+  // This might re-entrantly invalidate 'machine', which is a reference
+  // into Protos.
+  newMachine->initWithGenericSignature(sig);
+  return newMachine;
 }
 
 bool RewriteContext::isRecursivelyConstructingRequirementMachine(
@@ -484,14 +487,19 @@ RequirementMachine *RewriteContext::getRequirementMachine(
         llvm::errs() << " " << proto->getName();
       abort();
     }
-  } else {
-    // Construct a requirement machine from the structural requirements of
-    // the given set of protocols.
-    machine = new RequirementMachine(*this);
-    machine->initWithProtocols(component.Protos);
+
+    return machine;
   }
 
-  return machine;
+  // Construct a requirement machine from the structural requirements of
+  // the given set of protocols.
+  auto *newMachine = new RequirementMachine(*this);
+  machine = newMachine;
+
+  // This might re-entrantly invalidate 'machine', which is a reference
+  // into Protos.
+  newMachine->initWithProtocols(component.Protos);
+  return newMachine;
 }
 
 /// We print stats in the destructor, which should get executed at the end of