Add more context to not_a_type diagnostics

[natebosch]

Some time back the analyzer improved argument_type_not_assignable (and I'd imagine others) diagnostics to include more context about the types. Specifically it will say something like "where WhateverType is defined in URI".

Can we add similar context to not_a_type diagnostics? There should be a reference to a non-Type that we can point to.

[srawlins]

CC @bwilkerson