equiv_typedef: don't reject equivalence just for gensyms

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.

src/builtins.c

@@ -1395,7 +1395,7 @@ static int equiv_type(jl_value_t *ta, jl_value_t *tb)
  while (jl_is_unionall(a)) {
  jl_unionall_t *ua = (jl_unionall_t*)a;
  jl_unionall_t *ub = (jl_unionall_t*)b;
- if (!jl_egal(ua->var->lb, ub->var->lb) || !jl_egal(ua->var->ub, ub->var->ub) ||
+ if (!jl_types_egal(ua->var->lb, ub->var->lb) || !jl_types_egal(ua->var->ub, ub->var->ub) ||
  ua->var->name != ub->var->name)
  goto no;
  a = jl_instantiate_unionall(ua, (jl_value_t*)ub->var);

test/core.jl

@@ -7289,12 +7289,28 @@ end
 
 # issue #36104
 module M36104
+using Test
 struct T36104
  v::Vector{M36104.T36104}
 end
 struct T36104 # check that redefining it works, issue #21816
  v::Vector{T36104}
 end
+# with a gensymmed unionall
+struct Symmetric{T,S<:AbstractMatrix{<:T}} <: AbstractMatrix{T}
+ data::S
+ uplo::Char
+end
+struct Symmetric{T,S<:AbstractMatrix{<:T}} <: AbstractMatrix{T}
+ data::S
+ uplo::Char
+end
+@test_throws ErrorException begin
+ struct Symmetric{T,S<:AbstractMatrix{T}} <: AbstractMatrix{T}
+ data::S
+ uplo::Char
+ end
+end
 end
 @test fieldtypes(M36104.T36104) == (Vector{M36104.T36104},)
 @test_throws ErrorException("expected") @eval(struct X36104; x::error("expected"); end)