IMSBuildGlob should have a method that expands globs

[IMSBuildGlob](https://github.com/Microsoft/msbuild/blob/master/src/Build/Globbing/IMSBuildGlob.cs) currently knows how to match strings against itself.
It would be nice if it also had an `ExpandToFiles` method which hit the disk and expanded the glob into matching files, with the same behaviour that the engine is internally using. We didn't expose this functionality because there's no outside asks for it yet.

Implementation details:
- Since expansion is slow, the results should be cached, and maybe the method should also have an async counterpart
- A very easy implementation is to just call `EngineFileUtilities.GetFileList`. However, that method needlessly parses the glob again. A better implementation is to rework the internal static methods to also accept an already parsed glob. This poses some complications due to the different validation checks done by `EngineFileUtilities.GetFileList` and `FileMatcher.GetFiles`
- Behaviour on illegal filespecs (as per the engine today):
- `EngineFileUtilities.GetFileList` returns back the original illegal filespec
- `FileMatcher.GetFiles` return back the original illegal filespec if it's not excluded, otherwise returns empty