Clearer error message when unable to determine exact type parameter value in a method

[mortenpi]

It is possible to define methods where Julia is unable to determine a concrete value for a type parameter, leading to a UndefVarError:

julia> foo(::Ref{<:T}) where T = T
foo (generic function with 1 method)

julia> foo(Ref{String}())
String

julia> foo(Ref{AbstractString}())
ERROR: UndefVarError: `T` not defined

The error is confusing since, from a user's perspective, T is very much defined/declared in the where part of the method definition.

Rather, could we throw a dedicated AmbiguousTypeParameter error here (bikeshed, please)? Preferably with a docstring that would explain that it's possible to construct signatures where you can't uniquely figure out a type. And would also probably suggest avoiding unnecessary subtyping in method signature (like in the Ref{<:T} example above), since that is most likely an error on the user's part.

A couple of related issues I was able to find: #25307, #39277, #39280, #41841, #41728. It also caused some discussion on Zulip recently.

While for a different issue, the hints in #34393 might also help with the implementation here.

[vtjnash]

Also related to #25263