Improve display experience for unresolved symbols

[DanielRosenwasser]

Today if you have code like

function foo(x: Thing) {

}

and Thing is not resolved, quick info will just fill in Thing as any. It may be preferable to still display Thing, which could be achieved by synthesizing a new symbol on the fly. This is handy in partial semantic mode, where full program construction is still taking place in the background, or in scenarios like web editors where we'll never have the full project.

This should not affect declaration emit. Additionally, there's a risk that users find this confusing because it's occasionally useful to know that something is in a broken state. While anys are annoying, they help signal this.

The work involved here might be related to #38836