Support for MSBuildGlob serialization round-tripping

Given an MSBuildGlob (and presumably its variant classes), I need to be able to serialize and deserialize it. I cannot today because a given instance does not expose properties that match any constructor.

The scenario is RPC: I would like to send MSBuildGlob objects as arguments over RPC for VS caching.

This could be mitigated by simply adding a property whose value is the originally parsed string, since if I can acquire that, the remote process can simply pass it to Parse again.

Note that some globs are composite (e.g. `*.cs;*.vb;!notme.cs`) so however that is represented in msbuild glob classes needs to be reconstructible over RPC as well.