Change TypeVars in UnionAlls to print as var"" syntax (#34888)

This changes unnamed type variables to print as valid syntax, via the
`var""` printing macro.

For example, before this commit, `Vector{<:Bool}` printed as a
non-syntactic string:
```julia
julia> Vector{<:Bool}
Array{#s16,1} where #s16<:Bool
```

And now it prints as a valid, syntactic expression:
```julia
julia> Vector{<:Bool}
Array{var"#s46",1} where var"#s46"<:Bool
```

base/show.jl

@@ -1817,16 +1817,16 @@ function show(io::IO, tv::TypeVar)
     lb, ub = tv.lb, tv.ub
     if !in_env && lb !== Bottom
         if ub === Any
-            write(io, tv.name)
+            show_unquoted(io, tv.name)
             print(io, ">:")
             show_bound(io, lb)
         else
             show_bound(io, lb)
             print(io, "<:")
-            write(io, tv.name)
+            show_unquoted(io, tv.name)
         end
     else
-        write(io, tv.name)
+        show_unquoted(io, tv.name)
     end
     if !in_env && ub !== Any
         print(io, "<:")