[libc++] Add the _LIBCPP_HIDE_FROM_ABI_AFTER_V1 macro

Summary:
This macro allows hiding symbols from the ABI when the library is built
with an ABI version after ABI v1, which is currently the only stable ABI.
This commit defines `_LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY` to be
`_LIBCPP_HIDE_FROM_ABI_AFTER_V1`, meaning that symbols that were only
exported by the library for historical reasons are not exported anymore
in the unstable ABI.

Because of that, this commit is an ABI break for ABI v2. This ABI version
is not stable, however, so this should not be a problem.

Reviewers: EricWF, mclow.lists

Subscribers: christof, dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D49914

llvm-svn: 339012