Use __attribute__((internal_linkage)) when available.

Summary:
This patch has been a long time coming (Thanks @eugenis). It changes `_LIBCPP_INLINE_VISIBILITY` to use `__attribute__((internal_linkage))` instead of `__attribute__((visibility("hidden"), always_inline))`.

The point of `_LIBCPP_INLINE_VISIBILITY` is to prevent inline functions from being exported from both the libc++ library and from user libraries. This helps libc++ better manage it's ABI.
Previously this was done by forcing inlining and modifying the symbols visibility. However inlining isn't guaranteed and symbol visibility only affects shared libraries making this an imperfect solution.  `internal_linkage` improves this situation by making all symbols local to the TU they are emitted in, regardless of inlining or visibility. IIRC the effect of applying `__attribute__((internal_linkage))` to an inline function is the same as applying `static`.

For more information about the attribute see: http://lists.llvm.org/pipermail/cfe-dev/2015-October/045580.html

Most of the work for this patch was done by @eugenis.


Reviewers: mclow.lists, eugenis

Subscribers: eugenis, cfe-commits

Differential Revision: https://reviews.llvm.org/D24642

llvm-svn: 282345