[Sema] WitnessMatching: Anchor witness on the requirement declaration

(cherry picked from commit 4d501ef)

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -8552,7 +8552,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyConformsToConstraint(
         auto synthesizeConformance = [&]() {
           ProtocolConformanceRef synthesized(protocol);
           auto witnessLoc = getConstraintLocator(
-              /*anchor=*/{}, LocatorPathElt::Witness(witness));
+              locator.getAnchor(), LocatorPathElt::Witness(witness));
           SynthesizedConformances.insert({witnessLoc, synthesized});
           return recordConformance(synthesized);
         };

lib/Sema/TypeCheckProtocol.cpp

@@ -1154,8 +1154,8 @@ swift::matchWitness(WitnessChecker::RequirementEnvironmentCache &reqEnvCache,
     witnessType = witness->getInterfaceType();
     // FIXME: witness as a base locator?
     locator = cs->getConstraintLocator({});
-    witnessLocator = cs->getConstraintLocator({},
-                                              LocatorPathElt::Witness(witness));
+    witnessLocator =
+        cs->getConstraintLocator(req, LocatorPathElt::Witness(witness));
     if (witness->getDeclContext()->isTypeContext()) {
       openWitnessType = cs->getTypeOfMemberReference(
           selfTy, witness, dc, /*isDynamicResult=*/false,