Fix _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY to always have default visibility.

This prevent the symbols from being both externally available and hidden, which
causes them to be linked incorrectly. This is only a problem when the address
of the function is explicitly taken since it will always be inlined otherwise.

This patch fixes the issues that caused r285456 to be reverted, and can
now be reapplied.

llvm-svn: 285531