[Demangler] Fix OldRemangler's handling of Subscript nodes.

The Subscript node isn't laid-out the way the code in OldRemangler thinks,
which has the unfortunate result of causing crashes.

rdar://63410196

Author: al45tair

lib/Demangling/Demangler.cpp

@@ -3410,6 +3410,9 @@ NodePointer Demangler::demangleSubscript() {
   NodePointer LabelList = popFunctionParamLabels(Type);
   NodePointer Context = popContext();
 
+  if (!Type)
+    return nullptr;
+
   NodePointer Subscript = createNode(Node::Kind::Subscript);
   Subscript = addChild(Subscript, Context);
   addChild(Subscript, LabelList);

lib/Demangling/OldRemangler.cpp

@@ -874,7 +874,20 @@ void Remangler::mangleVariable(Node *node, EntityContext &ctx, unsigned depth) {
 
 void Remangler::mangleSubscript(Node *node, EntityContext &ctx,
                                 unsigned depth) {
-  mangleNamedAndTypedEntity(node, 'i', "", ctx, depth + 1);
+  assert(node->getNumChildren() >= 2);
+  Buffer << 'i';
+  mangleEntityContext(node->begin()[0], ctx, depth + 1);
+  if (node->getLastChild()->getKind() == Node::Kind::PrivateDeclName)
+    mangle(node->getLastChild(), depth + 1);
+
+  if (node->getNumChildren() >= 3
+      && node->begin()[1]->getKind() == Node::Kind::LabelList) {
+    auto LabelList = node->begin()[1];
+    auto Type = node->begin()[2];
+    mangleEntityType(applyParamLabels(LabelList, Type, Factory), ctx, depth + 1);
+  } else {
+    mangleEntityType(node->begin()[1], ctx, depth + 1);
+  }
 }
 
 void Remangler::mangleAccessor(Node *storageNode, StringRef accessorCode,

test/Demangle/Inputs/manglings.txt

@@ -412,3 +412,4 @@ $s4test10returnsOptyxycSgxyScMYccSglF ---> test.returnsOpt<A>((@Swift.MainActor
 $sSvSgA3ASbIetCyyd_SgSbIetCyyyd_SgD ---> (@escaping @convention(thin) @convention(c) (@unowned Swift.UnsafeMutableRawPointer?, @unowned Swift.UnsafeMutableRawPointer?, @unowned (@escaping @convention(thin) @convention(c) (@unowned Swift.UnsafeMutableRawPointer?, @unowned Swift.UnsafeMutableRawPointer?) -> (@unowned Swift.Bool))?) -> (@unowned Swift.Bool))?
 $s4test10returnsOptyxycSgxyScMYccSglF ---> test.returnsOpt<A>((@Swift.MainActor () -> A)?) -> (() -> A)?
 $s1t10globalFuncyyAA7MyActorCYiF ---> t.globalFunc(isolated t.MyActor) -> ()
+$sSIxip6foobarP ---> foobar in Swift.DefaultIndices.subscript : A

test/Demangle/remangle.swift

@@ -15,6 +15,9 @@ RUN: swift-demangle -remangle-objc-rt '$sSiStMSLB_p' | %FileCheck -check-prefix
 // CHECK-OLD3: Swift.related decl 'H' for partial apply forwarder
 RUN: swift-demangle -remangle-objc-rt '$ssTALHP' | %FileCheck -check-prefix CHECK-OLD3 %s
 
+// CHECK-OLD4: foobar in Swift.DefaultIndices.subscript : A
+RUN: swift-demangle -remangle-objc-rt '$sSIxip6foobarP' | %FileCheck -check-prefix CHECK-OLD4 %s
+
 // CHECK-GENERICEXT: Swift._ContiguousArrayStorage<(extension in Swift):Swift.FlattenSequence<StdlibCollectionUnittest.MinimalBidirectionalCollection<StdlibCollectionUnittest.MinimalBidirectionalCollection<Swift.Int>>>.Index>
 RUN: swift-demangle -remangle-objc-rt '$ss23_ContiguousArrayStorageCys15FlattenSequenceVsE5IndexVy24StdlibCollectionUnittest020MinimalBidirectionalH0VyAIySiGG_GGD' | %FileCheck -check-prefix CHECK-GENERICEXT %s