[libc++][chrono] Fixes month inc and dec operations.

The operator++, operator++(int), operator--, and operator--(int) need to
change the month to a valid value. The wording is specified in terms of
  operator+(const month& x, const months& y) noexcept;
which has the correct behavior. The aforementioned operators instead
used ++/-- on the internal value direction, resulting in incorrect
behaviour.

As a drive-by improve the unit tests:
- use the typical constexpr test method
- test whether the month is valid after the operations
- format the tests

Fixes: https://llvm.org/PR63912

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D155504