libstdc++: fix memory clobbering in std::vector [PR110879]

libstdc++: fix memory clobbering in std::vector [PR110879]

Fix ordering to prevent clobbering of class members by a call to deallocate
in _M_realloc_insert and _M_default_append.

Because of recent changes in _M_realloc_insert and _M_default_append,
calls to deallocate were ordered after assignment to class members of
std::vector (in the guard destructor), which is causing said members to
be call-clobbered.  This is preventing further optimization, the
compiler is unable to move memory read out of a hot loop in this case.

This patch reorders the call to before assignments by putting guard in
its own block. Plus a new testsuite for this case.  I'm not very happy
with the new testsuite, but I don't know how to properly test this.

	PR libstdc++/110879

libstdc++-v3/ChangeLog:

	* include/bits/vector.tcc (_M_realloc_insert): End guard
	lifetime just before assignment to class members.
	(_M_default_append): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/pr110879.C: New test.

Signed-off-by: Vladimir Palevich  <palevichva@gmail.com>