Put type attributes after class keyword

Summary:
Compiling `<functional>` in C++17 or higher mode results in:

```
functional:2500:1: warning: attribute '__visibility__' is ignored, place it after "class" to apply attribute to type declaration [-Wignored-attributes]
_LIBCPP_TYPE_VIS
^
__config:701:46: note: expanded from macro '_LIBCPP_TYPE_VIS'
#    define _LIBCPP_TYPE_VIS __attribute__ ((__visibility__("default")))
                                             ^
1 warning generated.
```

Fix it by putting the attribute after the `class` keyword.

Reviewers: EricWF, mclow.lists

Reviewed By: EricWF

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D43209

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@325027 91177308-0d34-0410-b5e6-96231b3b80d8