Parse GIR More Elegantly

[CodaFi]

The GIR parser is currently a hacked-together mess that is relying on the Silt parser to do the heavy lifting. If we're doing this anyways, let's just go all the way and give it some libSyntax nodes and a proper place in the parser.

While we're at it we should take a cue from SIL and let Silt and GIR terms live alongside one another.