[Mangling] Give protocol-conformance-ref an operator.

Due to some unfortunate refactoring, protocol-conformance-ref is a
nonterminal in the mangling grammar that doesn't have its own
operator:

```
protocol-conformance-ref ::= protocol module?
```

Both "module" and "protocol" can be an "identifier", which introduces
a mangling collision. Address the mangling collision by using the
operator "HP".

Fixes rdar://problem/46735592.

Author: DougGregor

docs/ABI/Mangling.rst

@@ -633,7 +633,7 @@ Property behaviors are implemented using private protocol conformances.
 ::
 
   concrete-protocol-conformance ::= type protocol-conformance-ref any-protocol-conformance-list 'HC'
-  protocol-conformance-ref ::= protocol module?
+  protocol-conformance-ref ::= protocol module? 'HP'
 
   any-protocol-conformance ::= concrete-protocol-conformance
   any-protocol-conformance ::= dependent-protocol-conformance

include/swift/Demangling/Demangler.h

@@ -458,7 +458,7 @@ class Demangler : public NodeFactory {
   NodePointer getDependentGenericParamType(int depth, int index);
   NodePointer demangleGenericParamIndex();
   NodePointer popProtocolConformance();
-  NodePointer popProtocolConformanceRef();
+  NodePointer demangleProtocolConformanceRef();
   NodePointer popAnyProtocolConformance();
   NodePointer demangleConcreteProtocolConformance();
   NodePointer popDependentProtocolConformance();

lib/AST/ASTMangler.cpp

@@ -2293,6 +2293,8 @@ void ASTMangler::appendProtocolConformanceRef(
   // are global anyway.
   if (isRetroactiveConformance(conformance))
     appendModule(conformance->getDeclContext()->getParentModule());
+
+  appendOperator("HP");
 }
 
 /// Retrieve the index of the conformance requirement indicated by the

lib/Demangling/Demangler.cpp

@@ -610,6 +610,7 @@ NodePointer Demangler::demangleOperator() {
       case 'C': return demangleConcreteProtocolConformance();
       case 'D': return demangleDependentProtocolConformanceRoot();
       case 'I': return demangleDependentProtocolConformanceInherited();
+      case 'P': return demangleProtocolConformanceRef();
       default:
         pushBack();
         pushBack();
@@ -1294,7 +1295,7 @@ NodePointer Demangler::popAnyProtocolConformance() {
   });
 }
 
-NodePointer Demangler::popProtocolConformanceRef() {
+NodePointer Demangler::demangleProtocolConformanceRef() {
   NodePointer module = popModule();
   NodePointer proto = popProtocol();
   auto protocolConformanceRef =
@@ -1309,7 +1310,7 @@ NodePointer Demangler::popProtocolConformanceRef() {
 
 NodePointer Demangler::demangleConcreteProtocolConformance() {
   NodePointer conditionalConformanceList = popAnyProtocolConformanceList();
-  NodePointer conformanceRef = popProtocolConformanceRef();
+  NodePointer conformanceRef = popNode(Node::Kind::ProtocolConformanceRef);
   NodePointer type = popNode(Node::Kind::Type);
   return createWithChildren(Node::Kind::ConcreteProtocolConformance,
                             type, conformanceRef, conditionalConformanceList);

lib/Demangling/Remangler.cpp

@@ -1657,6 +1657,7 @@ void Remangler::mangleProtocolConformanceRef(Node *node) {
   manglePureProtocol(node->getChild(0));
   if (node->getNumChildren() > 1)
     mangleChildNode(node, 1);
+  Buffer << "HP";
 }
 
 void Remangler::mangleConcreteProtocolConformance(Node *node) {

test/Demangle/Inputs/manglings.txt

@@ -301,7 +301,7 @@ _T0So13GenericOptionas8HashableSCsACP9hashValueSivgTW ---> {T:} protocol witness
 _T0So11CrappyColorVs16RawRepresentableSCMA ---> reflection metadata associated type descriptor __C.CrappyColor : Swift.RawRepresentable in __C_Synthesized
 $S28protocol_conformance_records15NativeValueTypeVAA8RuncibleAAMc ---> protocol conformance descriptor for protocol_conformance_records.NativeValueType : protocol_conformance_records.Runcible in protocol_conformance_records
 $SSC9SomeErrorLeVD ---> __C_Synthesized.related decl 'e' for SomeError
-$s20mangling_retroactive5test1yyAA2Z2V5InnerVy12RetroactiveB1XV_AG1YVAI0F1A1PAAyHCg_AkL1QAAyHCg0_GF ---> mangling_retroactive.test1(mangling_retroactive.Z2<RetroactiveB.X>.Inner<RetroactiveB.Y>) -> ()
+$s20mangling_retroactive5test0yyAA1ZVy12RetroactiveB1XVSiAE1YVAG0D1A1PAAHPyHCg_AiJ1QAAHPyHCg1_GF ---> mangling_retroactive.test0(mangling_retroactive.Z<RetroactiveB.X, Swift.Int, RetroactiveB.Y>) -> ()
 _T0LiteralAByxGxd_tcfC ---> _T0LiteralAByxGxd_tcfC
 _T0XZ ---> _T0XZ
 _TTSf0os___TFVs17_LegacyStringCore15_invariantCheckfT_T_ ---> function signature specialization <Arg[0] = Guaranteed To Owned and Exploded> of Swift._LegacyStringCore._invariantCheck() -> ()