[CSApply] Use ConcreteDeclRef for getParameterAt when possible

This is important if overload is variadic generic, in such cases
substitution map is used to determine actual parameter index.

(cherry picked from commit 377fd98)

Author: xedin

lib/Sema/CSApply.cpp

@@ -5490,7 +5490,7 @@ Solution::resolveLocatorToDecl(ConstraintLocator *locator) const {
 /// index. This looks through inheritance for inherited default args.
 static ConcreteDeclRef getDefaultArgOwner(ConcreteDeclRef owner,
                                           unsigned index) {
-  auto *param = getParameterAt(owner.getDecl(), index);
+  auto *param = getParameterAt(owner, index);
   assert(param);
   if (param->getDefaultArgumentKind() == DefaultArgumentKind::Inherited) {
     return getDefaultArgOwner(owner.getOverriddenDecl(), index);
@@ -6027,7 +6027,7 @@ ArgumentList *ExprRewriter::coerceCallArguments(
     };
 
     if (paramInfo.hasExternalPropertyWrapper(paramIdx)) {
-      auto *paramDecl = getParameterAt(callee.getDecl(), paramIdx);
+      auto *paramDecl = getParameterAt(callee, paramIdx);
       assert(paramDecl);
 
       auto appliedWrapper = appliedPropertyWrappers[appliedWrapperIndex++];