Use HIDE_FROM_ABI in the dylib

[mordante]

I think this should be HIDE_FROM_ABI. Otherwise, we'll have potential ODR violations when mixing different versions of libc++ via static archives. This seems to be a widespread pre-existing problem in the library after a quick survey.

This would force you to define it in the header because HIDE_FROM_ABI is ALWAYS_INLINE on GCC. I think it would be acceptable to ignore this issue until we've cleaned that up, since I feel like this is a very deep can of worms to open.

Originally posted by @ldionne in #74928 (comment)

[philnik777]

I'm pretty sure we've had this discussion before, but how could you ever link multiple static archives? They would provide the same public symbol multiple times.