compiler: Fix TypeVar/Vararg assertion in type limiting
The `__limit_type_size` function in typelimits.jl would incorrectly
pass non-Type arguments (specifically nested Vararges) to `_limit_type_size`,
triggering an assertion failure. This occurred when unwrapping a Vararg
type with `unwrapva` resulted in another Vararg or TypeVar, which was
not handled before falling through to `_limit_type_size`.
The fix adds a check after unwrapping the comparison type `c` to detect
if the result is still a Vararg or TypeVar, and recursively handles
these cases by calling `__limit_type_size` again. This ensures that
by the time we reach `_limit_type_size`, both arguments are proper Type
objects as expected by the assertion.
Fixes #59918
