Delay loading the witnesses of a protocol conformance.

Apart from being general compile-time goodness, this helps break a
circularity issue involving serialization cross-references and the
Clang importer.

The test is being added to validation-tests because it relies on
several levels of non-laziness in the compiler, all of which we'd
like to fix. It's making sure we don't regress here, but it isn't
actually verifying this change in particular.

rdar://problem/22364953

Swift SVN r31455

include/swift/Serialization/ModuleFile.h

@@ -599,6 +599,9 @@ public:
   virtual TypeLoc loadAssociatedTypeDefault(const AssociatedTypeDecl *ATD,
                                             uint64_t contextData) override;
 
+  virtual void finishNormalConformance(NormalProtocolConformance *conformance,
+                                       uint64_t contextData) override;
+
   Optional<BriefAndRawComment> getCommentForDecl(const Decl *D);
   Optional<BriefAndRawComment> getCommentForDeclByUSR(StringRef USR);
 
@@ -654,8 +657,8 @@ public:
   ProtocolConformance *readConformance(llvm::BitstreamCursor &Cursor);
 
   /// Read the given normal conformance from the current module file.
-  NormalProtocolConformance *readNormalConformance(
-                               serialization::NormalConformanceID id);
+  NormalProtocolConformance *
+  readNormalConformance(serialization::NormalConformanceID id);
 
   /// Reads a generic param list from \c DeclTypeCursor.
   ///