Errors on non-callable unions should be more specific

While working on #29011 I realized that the error we produce for unions of things which look callable but for which we cannot synthesize a signature for are lackluster. We report an error like:

Cannot use 'new' with an expression whose type lacks a call or construct signature.

when we could do better and say something like

Cannot use 'new' with an expression whose type lacks a call or construct signature. All members of union type {0} have call or construct signatures, but none are similar enough to resolve a call in a typesafe way.

which would make it far more clear what's going on in these scenarios.

Once #29011 is merged, the conditions can be even more specific, such as

Cannot use 'new' with an expression whose type lacks a call or construct signature. All members of union type {0} have call or construct signatures, but more than one has overloads, which prevents this call from being resolved in a typesafe way.

and

Cannot use 'new' with an expression whose type lacks a call or construct signature. All members of union type {0} have call or construct signatures, but more than one has type parameters, which prevents this call from being resolved in a typesafe way.

Start from the existing diagnostic for “Cannot use 'new' with an expression whose type lacks a call or construct signature” and review the related conditions described alongside #29011. Done means the reported union cases distinguish unsynthesizable signatures, multiple overloads, and multiple type parameters with the more specific messages shown.