[libcxx] Avoid overflows in the windows __libcpp_steady_clock_now()

As freq.QuadValue can be in the range of 10000000 to 19200000,
the multiplication before division makes the calculation overflow
and wrap to negative values every 16-30 minutes.

Instead count the whole seconds separately before adding the
scaled fractional seconds.

Add a testcase for steady_clock to check that the values returned for
now() compare as bigger than the zero time origin; this
corresponds to a testcase in Qt [1] [2] (that failed spuriously
due to this).

[1] https://bugreports.qt.io/browse/QTBUG-89539
[2] https://code.qt.io/cgit/qt/qtbase.git/tree/tests/auto/corelib/kernel/qdeadlinetimer/tst_qdeadlinetimer.cpp?id=f8de5e54022b8b7471131b7ad55c83b69b2684c0#n569

Differential Revision: https://reviews.llvm.org/D93456