Fix scope of hoisted signature-local variables

[Keno]

When we declare inner methods, e.g. the f in

function fs()
   f(lhs::Integer) = 1
   f(lhs::Integer, rhs::(local x=Integer; x)) = 2
   return f
end

we must hoist the definition of the (appropriately mangled) generic function f to top-level, including all variables that were used in the signature definition of f. This situation is a bit unique in the language because it uses inner function scope, but gets executed in toplevel scope. For example, you're not allowed to use a local of the inner function in the signature definition:

julia> function fs()
          local x=Integer
          f(lhs::Integer, rhs::x) = 2
          return f
       end
ERROR: syntax: local variable x cannot be used in closure declaration
Stacktrace:
 [1] top-level scope
   @ REPL[3]:1

In particular, the restriction is signature-local:

julia> function fs()
          f(rhs::(local x=Integer; x)) = 1
          f(lhs::Integer, rhs::x) = 2
          return f
       end
ERROR: syntax: local variable x cannot be used in closure declaration
Stacktrace:
 [1] top-level scope
   @ REPL[4]:1

There's a special intermediate form moved-local that gets generated for this definition. In c6c3d72, this form stopped getting generated for certain inner methods. I suspect this happened because of the incorrect assumption that the set of moved locals is being computed over all signatures, rather than being a per-signature property.

The result of all of this was that this is one of the few places where lowering still generated a symbol as the lhs of an assignment for a global (instead of globalref), because the code that generates the assignment assumes it's a local, but the later pass doesn't know this. Because we still retain the code for this from before we started using globalref consistently, this wasn't generally causing a problems, except possibly leaking a global (or potentially assigning to a global when this wasn't intended). However, in follow on work, I want to make use of knowing whether the LHS is a global or local in lowering, so this was causing me trouble.

Fix all of this by putting back the moved-local where it was dropped.

Fixes #56711