fix(@angular/cli): pass sourceMap to postcss

This is required per https://github.com/postcss/postcss-loader#sourcemap

Closes #8588

Author: Manduro

packages/@angular/cli/models/webpack-configs/styles.ts

@@ -177,7 +177,8 @@ export function getStylesConfig(wco: WebpackConfigOptions) {
       options: {
         // A non-function property is required to workaround a webpack option handling bug
         ident: 'postcss',
-        plugins: postcssPluginCreator
+        plugins: postcssPluginCreator,
+        sourceMap: cssSourceMap
       }
     }
   ];