[Remote] Add protocol conformance demangling support.

MetadataReader should expose the builder's protocol conformance related types,
and should provide a method to demangle a mangled protocol conformance.

rdar://46280860

Author: al45tair

include/swift/Remote/MetadataReader.h

@@ -165,6 +165,9 @@ class MetadataReader {
   using BuiltTypeDecl = typename BuilderType::BuiltTypeDecl;
   using BuiltProtocolDecl = typename BuilderType::BuiltProtocolDecl;
   using BuiltRequirement = typename BuilderType::BuiltRequirement;
+  using BuiltProtocolConformance = typename BuilderType::BuiltProtocolConformance;
+  using BuiltProtocolConformanceDecl =
+    typename BuilderType::BuiltProtocolConformanceDecl;
   using StoredPointer = typename Runtime::StoredPointer;
   using StoredSignedPointer = typename Runtime::StoredSignedPointer;
   using StoredSize = typename Runtime::StoredSize;
@@ -472,6 +475,12 @@ class MetadataReader {
     return swift::Demangle::decodeMangledType(Builder, Node);
   }
 
+  /// Given a demangle tree, attempt to turn it into a protocol conformance
+  TypeLookupErrorOr<typename BuilderType::BuiltProtocolConformance>
+  decodeMangledProtocolConformance(NodePointer Node) {
+    return swift::Demangle::decodeMangledProtocolConformance(Builder, Node);
+  }
+
   /// Get the remote process's swift_isaMask.
   llvm::Optional<StoredPointer> readIsaMask() {
     auto encoding = getIsaEncoding();