libstdc++: Move start_lifetime_as functions to bits/stl_construct.h [PR106658]

libstdc++: Move start_lifetime_as functions to bits/stl_construct.h [PR106658]

This allows inplace_vector to use these functions without including the entire
<memory> header.

Preprocessor checks are changed to use __glibcxx macros, so new functions are
available outside memory header, that exports __cpp_lib macros.

	PR libstdc++/106658

libstdc++-v3/ChangeLog:

	* include/bits/stl_construct.h (std::start_lifetime_as_array)
	(std::start_lifetime_as): Moved from std/memory, with update
	to guard macros.
	* include/std/memory (std::start_lifetime_as_array)
	(std::start_lifetime_as): Moved to bits/stl_construct.h.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>