[analyzer][NFC] Consolidate the inner representation of CallDescriptions

`CallDescriptions` have a `RequiredArgs` and `RequiredParams` members,
but they are of different types, `unsigned` and `size_t` respectively.
In the patch I use only `unsigned` for both, that should be large enough
anyway.
I also introduce the `MaybeUInt` type alias for `Optional<unsigned>`.

Additionally, I also avoid the use of the //smart// less-than operator.

  template <typename T>
  constexpr bool operator<=(const Optional<T> &X, const T &Y);

Which would check if the optional **has** a value and compare the data
only after. I found it surprising, thus I think we are better off
without it.

Reviewed By: martong, xazax.hun

Differential Revision: https://reviews.llvm.org/D113594