[libc++] NFCI: Refactor __shared_ptr_emplace

This is the first of a series of patches leading up to the implementation
of P0674r1, i.e. array support in allocate_shared. I am splitting this
up into multiple patches because the overall change is very tricky and
I want to isolate potential breakage.

GitOrigin-RevId: 092e8a7ea3652c418400275746c1800d31390008
1 file changed