libstdc++: Make chrono::parse accept out-of-range values that aren't needed [PR126364]

When parsing a time with %R or %T we should ignore out of range hours
and minutes if the type being parsed doesn't need them, e.g. when
parsing a chrono::year_month_day from "2026-07-29 99:99:99" we do not
set failbit, and should continue parsing after the invalid hours and
minutes.

Because we were short circuiting as soon as we saw "99" (in either
field) we didn't parse to the end of the %R or %T field, and then could
set failbit if there were any subsequent characters or flags to parse.

The fix is to only short-circuit when setting failbit, and continue
parsing otherwise.

With this change, we no longer hit the 'break' when __read_unsigned(2)
returns -1 (e.g. because the input was non-numeric) unless we're
parsing a type that needs the %R or %T value. But that's OK, because
__read_unsigned sets failbit when it returns -1 and so the next
__read_chr or __read_unsigned will fail without extracting more
characters, and we'll break there instead. So there's no change in
observable behaviour for non-numeric inputs, only for out-of-range
numeric inputs.

libstdc++-v3/ChangeLog:

	PR libstdc++/126364
	* include/bits/chrono_io.h (_Parser::operator()) <R>: Only break
	early when setting failbit.
	* testsuite/std/time/parse/126364.cc: New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>