Provide a better display experience for unresolved values and types

[DanielRosenwasser]

Today, an unresolved type, or the type of an unresolved variable will look something like any. Internally, these resolve to a well-known error type that semantically acts as any.

However, in #37713, we may not have a full picture of the world because we're limited to a single file.

I think we should consider doing two things

1. In the syntactic mode, the type of any unresolved symbol should be of a new any-like type called loading.
2. anywhere else, usage of an unresolved type should "stick around" and be recognized as a type reference to an extremely permissive type. This could be implemented through something similar to Ensure find-all-refs works on unresolved locals #38836

According to @CyrusNajmabadi, Roslyn already does something really similar to this.