hash malfunction for x::UnionAll

[goulf-3m]

For Set, unique to work, correct hashing is required.
Before isequal is used for comparison, hash is used to find the slot.
Thus, equal values should have equal hashes, so as to meet at the same slot.

However, for UnionAll types, hash is unstable: equal values have different hashes:

julia> a=Array{T} where T
Array
julia> b=Array{T2} where T2
Array
julia> a==b, a===b, hash(a), hash(b)
(true, false, 0x3e735c1c362de00f, 0x6ba6f097cee26fae)
julia> unique([a,b])
2-element Vector{UnionAll}:
 Array
 Array

versioninfo():

Julia Version 1.9.4
Commit 8e5136fa297 (2023-11-14 08:46 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 16 × AMD Ryzen 9 6900HX with Radeon Graphics
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-14.0.6 (ORCJIT, znver3)
  Threads: 1 on 16 virtual cores

[N5N3]

Fixed by #49636.