Fixes #5693

When there's code like this:

def foo(**opts)
  opts
end

foo(x: 1, y: 2)

the compiler transforms it to something like this (call, per call):

def foo:x:y(x, y)
  opts = {x: x, y: y}
  # next comes the original def's body, in this case just "opts"
  opts
end

foo(x: 1, y: 2)

The problem with that approach is that this will not work correctly if the method already defined x or y as local variables. This:

def foo(**opts)
  x = 1
  opts
end

foo(x: 1, y: 2)

gets expanded to:

def foo:x:y(x, y)
  opts = {x: x, y: y}
  x = 1
  opts
end

foo(x: 1, y: 2)

This isn't usually a problem unless x is declared as a local variable with x : Type, or if x is captured in a closure.

The solution in this PR changes the expansion to be:

def foo:x:y(x __temp_1, y __temp_2)
  opts = {x: __temp_1, y: __temp_2}
  x = 1
  opts
end

foo(x: 1, y: 2)

That way there's no clash.

(well, there could be a clash with those temp names, but variables that start with underscore are reserved for the compiler, more so ones with double underscore (this is used all over the compiler, not just here))