[Serialization] witness_method should reference existing conformances.

Part of a series of commits to remove redundantly-serialized conformances.

Swift SVN r22196

lib/Serialization/SILFormat.h

@@ -136,6 +136,7 @@ namespace sil_block {
     SIL_WITNESS_ASSOC_PROTOCOL,
     SIL_WITNESS_ASSOC_ENTRY,
     SIL_GENERIC_OUTER_PARAMS,
+    SIL_INST_WITNESS_METHOD,
 
     // We also share these layouts from the decls block. Their enumerators must
     // not overlap with ours.
@@ -346,6 +347,20 @@ namespace sil_block {
     SIL_GENERIC_OUTER_PARAMS,
     DeclIDField // The decl id of the outer param if any.
   >;
+
+  using SILInstWitnessMethodLayout = BCRecordLayout<
+    SIL_INST_WITNESS_METHOD,
+    TypeIDField,           // result type
+    SILTypeCategoryField,
+    BCFixed<1>,            // volatile?
+    TypeIDField,           // lookup type
+    SILTypeCategoryField,
+    DeclIDField,           // conformance proto
+    TypeIDField,           // conformance type
+    ModuleIDField,         // conformance module
+    BCArray<ValueIDField>  // SILDeclRef
+    // may be trailed by an inline protocol conformance
+  >;
 }
 
 } // end namespace serialization