[libc++] Fix _LIBCPP_EXPORTED_FROM_ABI when visibility annotations are disabled
[libc++] Fix _LIBCPP_EXPORTED_FROM_ABI when visibility annotations are disabled

Fixes a bug where functions would get exported when building with
-fvisibility=hidden and defining _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS. No
visibility annotations should be added in this case.

The new logic for _LIBCPP_EXPORTED_FROM_ABI matches that of the other visibility
annotations around it.

Differential Revision: https://reviews.llvm.org/D55664



git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@349080 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx