libstdc++: Fix -Wstringop-overread warning in std::vector<bool> [PR114758]

libstdc++: Fix -Wstringop-overread warning in std::vector<bool> [PR114758]

As in r13-4393-gcca06f0d6d76b0 and a few other commits, we can avoid
bogus warnings in std::vector<bool> by hoisting some loads to before the
allocation that calls operator new. This means that the compiler has
enough info to remove the dead branches that trigger bogus warnings.

On trunk this is only needed with -fno-assume-sane-operators-new-delete
but it will help on the branches where that option doesn't exist.

libstdc++-v3/ChangeLog:

	PR libstdc++/114758
	* include/bits/vector.tcc (vector<bool, A>::_M_fill_insert):
	Hoist loads of begin() and end() before allocation.
	* testsuite/23_containers/vector/bool/capacity/114758.cc: New
	test.

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