Suppress -Wctad-maybe-unsupported on types w/o deduction guides.

There are a handful of standard library types that are intended
to support CTAD but don't need any explicit deduction guides to
do so.

This patch adds a dummy deduction guide to those types to suppress
-Wctad-maybe-unsupported (which gets emitted in user code).

llvm-svn: 367770
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: fcd549a7d8284a8e7c763fee3da2206acd8cdc4f
diff --git a/include/functional b/include/functional
index 865a281..dd65f1b 100644
--- a/include/functional
+++ b/include/functional
@@ -3067,7 +3067,8 @@
     _ForwardIterator __first_;
     _ForwardIterator __last_;
     _BinaryPredicate __pred_;
-    };
+};
+_LIBCPP_CTAD_SUPPORTED_FOR_TYPE(default_searcher);
 
 #endif // _LIBCPP_STD_VER > 14