[libc++] Replace uses of _LIBCPP_ALWAYS_INLINE by _LIBCPP_INLINE_VISIBILITY

Summary:
We never actually mean to always inline a function -- all the uses of
the macro I could find are actually attempts to control the visibility
of symbols. This is better described by _LIBCPP_INLINE_VISIBILITY, which
is actually always defined the same.

This change is orthogonal to the decision of what we're actually going
to do with _LIBCPP_INLINE_VISIBILITY -- it just simplifies things by
having one canonical way of doing things.

Reviewers: EricWF

Subscribers: christof, llvm-commits, dexonsmith, erikvanderpoel, mclow.lists

Differential Revision: https://reviews.llvm.org/D48892

llvm-svn: 336369