Expose lexing functions for XMLDoc extension

I'm migrating F# XMLDoc extension to VS 2012 and VS 2013 as part of https://github.com/fsprojects/FSharpVSPowerTools.

Here are the minimal changes in order that F# XMLDoc can be migrated to use FSharp.Compiler.Service (see https://github.com/fsprojects/FSharpVSPowerTools/issues/4 for more details).

If this is merged, please publish a new version on NuGet.