[Bug Report] Top-level selectors may not contain the parent selector "&"

### Element UI version
2.15.8

### OS/Browsers version
mac

### Vue version
2.5.22

### Reproduction Link
https://codepen.io/ziyoung/pen/LKNBqB

### Steps to reproduce
vue2.5.2
sass1.53.0
sass-loader8.0.2
element-ui 2.15.8
运行时会提示
╷
167 │ &.#{$state-prefix $state} {
│ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
╵
node_modules/element-ui/packages/theme-chalk/src/mixins/mixins.scss

### What is Expected?
vue项目使用sass( 不是 node-sass ) sass-loader 可以运行正常

### What is actually happening?
./src/assets/css/global/global.scss (./node_modules/css-loader/dist/cjs.js??ref--10-1!./node_modules/postcss-loader/lib??ref--10-2!./node_modules/sass-loader/dist/cjs.js??ref--10-3!./node_modules/sass-resources-loader/lib/loader.js??ref--10-4!./src/assets/css/global/global.scss)
Module build failed (from ./node_modules/sass-loader/dist/cjs.js):
SassError: Top-level selectors may not contain the parent selector "&".
╷
167 │ &.#{$state-prefix $state} {
│ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
╵
node_modules/element-ui/packages/theme-chalk/src/mixins/mixins.scss 167:5 when()