Previously, `absl::Condition` incorrectly used the same (non-`const`)
pointer-to-method type when wrapping both `const` and non-`const` methods.

Unfortunately, this is undefined behavior according to `[expr.reinterpret.cast]`
in the C++ standard:

> The effect of calling a function through a pointer to a function type that is
> not the same as the type used in the definition of the function is undefined.

This fixes the UB.

PiperOrigin-RevId: 591981682
Change-Id: Iaca955346699417232383d3a1800ea9b82ea5761