[ConstraintSystem] Use locator from result of cast rather than source.

Be consistent with CoerceExpr and for the constraint we create use the
locator from the result of the expression rather than the locator from
the source of the expression.

Author: rudkx

lib/Sema/CSGen.cpp

@@ -2512,7 +2512,7 @@ namespace {
       expr->getCastTypeLoc().setType(toType, /*validated=*/true);
 
       auto fromType = CS.getType(fromExpr);
-      auto locator = CS.getConstraintLocator(fromExpr);
+      auto locator = CS.getConstraintLocator(expr);
 
       // The source type can be checked-cast to the destination type.
       CS.addConstraint(ConstraintKind::CheckedCast, fromType, toType, locator);
@@ -2560,7 +2560,7 @@ namespace {
       expr->getCastTypeLoc().setType(toType, /*validated=*/true);
 
       auto fromType = CS.getType(fromExpr);
-      auto locator = CS.getConstraintLocator(fromExpr);
+      auto locator = CS.getConstraintLocator(expr);
       CS.addConstraint(ConstraintKind::CheckedCast, fromType, toType, locator);
       return OptionalType::get(toType);
     }