Make possible to don't provide hybrid error handling.

[viboes]

Boost.Chrono provides by default an hybrid error handling for the Clock::now() function, implementing the following prototype

Clock::now(system::error_code&= boost::throws());

Now that the standard has accepted the prototype

Clock::now() noexcept;

the aforementioned prototype can not be noexcept, as the use of boost::throws() force the throw exception on error.

We need to overload the function as follows:

Clock::now() noexcept; Clock::now(system::error_code&);

This lost the advantages of the announced by the hybrid error handling design, as we are forced to have two implementations.

In addition, while most of the uses of now(ec) expect that there is no exception, the prototype can not state it using noexcept, if the boost::throws() semantic is taken in account.

Due to this split, we can now make Boost.Chrono a real header-only library if we admit to renounce to use the error_code interface.

The library could add a macro that prevent the inclusion of these prototype, as e.g. BOOST_CHRONO_DONT_PROVIDE_HYBRID_ERROR_HANDLING.

[viboes]

Merged with #74801