[CoroutineAccessors] Set field on RequirementMatch

The WitnessSubstitutions field must be set to the appropriate
substitution map.

Author: nate-chandler

lib/Sema/TypeCheckProtocol.cpp

@@ -6850,6 +6850,7 @@ void TypeChecker::inferDefaultWitnesses(ProtocolDecl *proto) {
         RequirementEnvironment reqEnv(proto, reqSig, proto, nullptr, nullptr);
         auto match =
             RequirementMatch(asd, MatchKind::ExactMatch, asdTy, reqEnv);
+        match.WitnessSubstitutions = reqEnv.getRequirementToWitnessThunkSubs();
         checker.recordWitness(asd, match);
       }
     }

test/SILGen/coroutine_accessors.swift

@@ -235,3 +235,13 @@ class OverridableReader : GettableTitle {
     }
   }
 }
+
+// CHECK-LABEL: sil_default_witness_table ReadableField {
+// CHECK-NEXT:    no_default
+// CHECK-NEXT:    method #ReadableField.field!read2 
+// CHECK-SAME:        : @$s19coroutine_accessors13ReadableFieldP5fieldSivy
+// CHECK-NEXT:  }
+public protocol ReadableField {
+  @_borrowed
+  var field: Int { get }
+}