Restrict LinearAlgebra.onedefined to concrete number types

[jishnub]

This makes onedefined more conservative, so that now

julia> LinearAlgebra.onedefined(Number)
false

julia> LinearAlgebra.onedefined(Union{Float64,ComplexF64})
false

In general, we cannot guarantee that oneunit works with abstract or union types. After this, the following contrived example works correctly instead of throwing an error:

julia> using Symbolics

julia> @variables x
1-element Vector{Num}:
 x

julia> A = Bidiagonal{Real}([1,2], [x], :U)
2×2 Bidiagonal{Real, Vector{Real}}:
 1  x
 ⋅  2

julia> A \ [1,2]
2-element Vector{Any}:
 1 - x
     1.0

This change widens certain inferred types. E.g., on master:

julia> Bidiagonal{Number}(ones(4), ones(3), :U) \ ones(4)
4-element Vector{Float64}:
 0.0
 1.0
 0.0
 1.0

whereas, in this PR, this returns a Vector{Any}. This may be acceptable, though, if one starts with abstract types.

[dkarrasch]

I guess this would be obsolete after #52464?

[jishnub]

Yes, I'll close this for now