[libc++] Give extern templates default visibility on gcc
Contrary to the current visibility macro documentation, it appears that
gcc does handle visibility attribute on extern templates correctly, e.g.
https://godbolt.org/g/EejuV7. We need this so that extern template
instantiations of classes not marked _LIBCPP_TEMPLATE_VIS (e.g.
__vector_base_common) are correctly exported with gcc when building with
hidden visibility.
Reviewed By: ldionne
Differential Revision: https://reviews.llvm.org/D35388
GitOrigin-RevId: 0066a09579ca90f60cb1947691e5a441f9f57a5d
2 files changed
