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

llvm-svn: 325027
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 1b1b80f45fdd116affb6bb7d0e08786cc5fee996
diff --git a/include/functional b/include/functional
index 69bcada..e45beb8 100644
--- a/include/functional
+++ b/include/functional
@@ -2497,8 +2497,7 @@
 
 // default searcher
 template<class _ForwardIterator, class _BinaryPredicate = equal_to<>>
-_LIBCPP_TYPE_VIS
-class default_searcher {
+class _LIBCPP_TYPE_VIS default_searcher {
 public:
     _LIBCPP_INLINE_VISIBILITY
     default_searcher(_ForwardIterator __f, _ForwardIterator __l,