Adorn __call_once_proxy with `inline` and `_LIBCPP_INLINE_VISIBILITY`

As per discussion with mclow and EricWF on irc, this is small and
simple enough to deserve being inlined.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@291497 91177308-0d34-0410-b5e6-96231b3b80d8