[Runtime] Add a means to obtain a conformance descriptor.

For protocol conformance demangling, we will need to be able to fetch a
ProtocolConformanceDescriptor given a protocol and a type that might conform
to that protocol.  Add a function to do that, as well as a cache for that
function's results.

rdar://46280860

Author: al45tair

stdlib/public/CompatibilityOverride/CompatibilityOverrideRuntime.def

@@ -217,6 +217,12 @@ OVERRIDE_FOREIGN(dynamicCastForeignClassUnconditional, const void *, , , swift::
 
 #endif
 
+OVERRIDE_PROTOCOLCONFORMANCE(getConformanceDescriptor,
+                             const ProtocolConformanceDescriptor *, , , swift::,
+                             (const Metadata * const type,
+                              const ProtocolDescriptor *protocol),
+                             (type, protocol))
+
 #undef OVERRIDE
 #undef OVERRIDE_METADATALOOKUP
 #undef OVERRIDE_CASTING

stdlib/public/runtime/Private.h

@@ -643,6 +643,17 @@ class TypeInfo {
   id _quickLookObjectForPointer(void *value);
 #endif
 
+  /// Retrieve a conformance descriptor given a type and protocol.
+  /// Note that this DOES NOT take into account generic arguments.
+  ///
+  /// \param type The type we're considering.
+  /// \param protocol The protocol to which the type may conform.
+  ///
+  /// \returns a protocol conformance descriptor, or nullptr if none found.
+  const ProtocolConformanceDescriptor *
+  swift_getConformanceDescriptor(const Metadata * const type,
+                                 const ProtocolDescriptor *protocol);
+
 } // end namespace swift
 
 #endif /* SWIFT_RUNTIME_PRIVATE_H */