LibraryElement: expose the “source” of the language version

Right now, we have

  /// The major component of the language version for this library.
  int get languageVersionMajor;

  /// The minor component of the language version for this library.
  int get languageVersionMinor;

This is great, but we also need to know the source of the version – specifically, we need to know if the version is from a language version comment – // @dart=2.9.

This will allow us to generate parts that match any existing comment on a library. See also #42456