[libc++] Declare <compare> operators with the proper visibility attribute

Summary:
Many operators in <compare> were _defined_ with the proper visibility attribute,
but they were _declared_ without any. This is not a problem until we change the
definition of _LIBCPP_INLINE_VISIBILITY to something that requires the
declaration to be decorated.

I also marked `strong_equality::operator weak_equality()` as
`_LIBCPP_INLINE_VISIBILITY`, since it seems like it had been forgotten.

This came up while trying to get rid of `__attribute__((__always_inline__))`
in favor of `__attribute__((internal_linkage))`.

Reviewers: EricWF, mclow.lists

Subscribers: christof, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D49104

llvm-svn: 336665