[Runtime] Add dummy "module name" parameter to swift::_conformsToSwiftProtocol.

We know we're going to want this parameter to handle lookup of
retroactive conformances when we've been given a module, so add it
now because we want this to be an overridable entry point.

Author: DougGregor

stdlib/public/runtime/Private.h

@@ -511,7 +511,8 @@ class TypeInfo {
   /// returning the appropriate protocol conformance descriptor when it does.
   const ProtocolConformanceDescriptor *
   _conformsToSwiftProtocol(const Metadata * const type,
-                           const ProtocolDescriptor *protocol);
+                           const ProtocolDescriptor *protocol,
+                           StringRef module);
 
   /// Retrieve an associated type witness from the given witness table.
   ///

stdlib/public/runtime/ProtocolConformance.cpp

@@ -534,7 +534,8 @@ namespace {
 
 const ProtocolConformanceDescriptor *
 swift::_conformsToSwiftProtocol(const Metadata * const type,
-                                const ProtocolDescriptor *protocol) {
+                                const ProtocolDescriptor *protocol,
+                                StringRef module) {
   auto &C = Conformances.get();
 
   // See if we have a cached conformance. The ConcurrentMap data structure
@@ -600,7 +601,7 @@ swift::_conformsToSwiftProtocol(const Metadata * const type,
 static const WitnessTable *
 swift_conformsToProtocolImpl(const Metadata * const type,
                              const ProtocolDescriptor *protocol) {
-  auto description = _conformsToSwiftProtocol(type, protocol);
+  auto description = _conformsToSwiftProtocol(type, protocol, StringRef());
   if (!description)
     return nullptr;