Fix source maps of variant utilities that come from an @layer rule

[thecrypticace]

If you have a utility like the following:

@layer utilities {
  .foo {
    color: red;
  }
}

And your content contains foo or hover:foo then the generated CSS has correct source map information. However, in the case of a utility with a variant that generates an at-rule (e.g. lg:foo) we were incorrectly overwriting this information for all rules inside the at-rule.

Now, after this PR, these source locations are correctly preserved.

Fixes #12464