[Diagnostics] Adjust requirement failures to retrieve conformance information from locator

Conditional conformances previously relied on `CheckedConformances`
cache to retrieve information about conditional requirements from
base conformance, but that is not really necessary anymore since
`ConformanceRequirement` element could provide all required info.

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -205,23 +205,14 @@ const Requirement &RequirementFailure::getRequirement() const {
 
 ProtocolConformance *RequirementFailure::getConformanceForConditionalReq(
     ConstraintLocator *locator) {
-  auto &solution = getSolution();
   auto reqElt = locator->castLastElementTo<LocatorPathElt::AnyRequirement>();
   if (!reqElt.isConditionalRequirement())
     return nullptr;
 
-  auto path = locator->getPath();
-  auto *conformanceLoc = getConstraintLocator(getRawAnchor(), path.drop_back());
-
-  auto result = llvm::find_if(
-      solution.Conformances,
-      [&](const std::pair<ConstraintLocator *, ProtocolConformanceRef>
-              &conformance) { return conformance.first == conformanceLoc; });
-  assert(result != solution.Conformances.end());
-
-  auto conformance = result->second;
-  assert(conformance.isConcrete());
-  return conformance.getConcrete();
+  auto conformanceRef =
+    locator->findLast<LocatorPathElt::ConformanceRequirement>();
+  assert(conformanceRef && "Invalid locator for a conditional requirement");
+  return conformanceRef->getConformance();
 }
 
 ValueDecl *RequirementFailure::getDeclRef() const {

lib/Sema/ConstraintSystem.cpp

@@ -5181,22 +5181,12 @@ static Optional<Requirement> getRequirement(ConstraintSystem &cs,
     return None;
 
   if (reqLoc->isConditionalRequirement()) {
-    auto path = reqLocator->getPath();
-    auto *conformanceLoc =
-        cs.getConstraintLocator(reqLocator->getAnchor(), path.drop_back());
-
-    auto conformances = cs.getCheckedConformances();
-    auto result = llvm::find_if(
-        conformances,
-        [&conformanceLoc](
-            const std::pair<ConstraintLocator *, ProtocolConformanceRef>
-                &conformance) { return conformance.first == conformanceLoc; });
-    assert(result != conformances.end());
-
-    auto conformance = result->second;
-    assert(conformance.isConcrete());
-
-    return conformance.getConditionalRequirements()[reqLoc->getIndex()];
+    auto conformanceRef =
+        reqLocator->findLast<LocatorPathElt::ConformanceRequirement>();
+    assert(conformanceRef && "Invalid locator for a conditional requirement");
+
+    auto conformance = conformanceRef->getConformance();
+    return conformance->getConditionalRequirements()[reqLoc->getIndex()];
   }
 
   if (auto openedGeneric =