Sema: Address -Wunused-but-set-variable warnings.

Author: tshortli

lib/Sema/CSGen.cpp

@@ -194,7 +194,7 @@ namespace {
       }
 
       if (auto DRE = dyn_cast<DeclRefExpr>(expr)) {
-        if (auto varDecl = dyn_cast<VarDecl>(DRE->getDecl())) {
+        if (isa<VarDecl>(DRE->getDecl())) {
           if (CS.hasType(DRE)) {
             LTI.collectedTypes.insert(CS.getType(DRE).getPointer());
           }

lib/Sema/CSSimplify.cpp

@@ -10834,7 +10834,7 @@ static ConstraintFix *validateInitializerRef(ConstraintSystem &cs,
     // which means MetatypeType has to be added after finding a type variable.
     if (baseLocator->isLastElement<LocatorPathElt::MemberRefBase>())
       baseType = MetatypeType::get(baseType);
-  } else if (auto *keyPathExpr = getAsExpr<KeyPathExpr>(anchor)) {
+  } else if (getAsExpr<KeyPathExpr>(anchor)) {
     // Key path can't refer to initializers e.g. `\Type.init`
     return AllowInvalidRefInKeyPath::forRef(cs, baseType, init, locator);
   }

lib/Sema/TypeCheckAttr.cpp

@@ -5154,8 +5154,7 @@ void AttributeChecker::checkBackDeployedAttrs(
 
     // Unavailable decls cannot be back deployed.
     auto backDeployedDomain = AvailabilityDomain::forPlatform(Attr->Platform);
-    if (auto unavailableDomain =
-            availability.containsUnavailableDomain(backDeployedDomain)) {
+    if (availability.containsUnavailableDomain(backDeployedDomain)) {
       auto domainForDiagnostics = backDeployedDomain;
       llvm::VersionTuple ignoredVersion;