Since the application of P0439R0, the library wording has been partially
updated to use the new scoped enumaration values whenever memory order
constants are required, and partially retains use of the inline
constants supplied for back compatibility.

This patch conistently uses the memory_order::enumerators, retaining
the old name only for their declaration, the index, and for Annex C
wording on C++17 compatibility, which /should/ use the old spelling.

The atomic API for shared_ptr has a couple of returns clauses bleeding
their trailing period beyond the margin, so revert the whole clause
for consistency, allowing the main text edits to proceed.