inference: restrict type of type parameter from `Vararg` (#51449)
Inference has been able to restrict type of `Vararg` type parameter `N`
to `Int` for cases like `func(..., ::Vararg{T,N}) where {T,N}`, but this
refinement was not available for signatures like
`func(::Tuple{Vararg{T,N}}) where {T,N}`.
This commit allows the later case to be inferred as well. Now the
following kind of case will be inferred, e.g.:
```julia
julia> function sub2ind_gen_fallback(dims::NTuple{N,Int}, I) where N # N is knonw to be ::Int
ind = I[N] - 1
for i = (N - 1):-1:1
ind = I[i] - 1 + dims[i]*ind
end
return ind + 1
end;
julia> only(Base.return_types(sub2ind_gen_fallback, (NTuple,Tuple{Vararg{Int}})))
Int64
```
