equiv_typedef: don't reject equivalence just for gensyms (#39778)
This fixes the following:
```
julia> struct Symmetric{T,S<:AbstractMatrix{<:T}} <: AbstractMatrix{T}
data::S
uplo::Char
end
julia> struct Symmetric{T,S<:AbstractMatrix{<:T}} <: AbstractMatrix{T}
data::S
uplo::Char
end
ERROR: invalid redefinition of constant Symmetric
```
`Core._equiv_typedef` returns false which ends up triggering the error.
