[libc++][test] Properly mark libc++-only XFAILs

These tests PASS on libstdc++ and MSVC.

libcxx/test/std/strings/c.strings/cuchar.pass.cpp

@@ -6,7 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// XFAIL: *
+// XFAIL: libc++
 
 // <cuchar>
 

libcxx/test/std/utilities/function.objects/func.search/func.search.bm/default.pass.cpp

@@ -7,7 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++98, c++03, c++11, c++14
-// XFAIL: c++17, c++2a
+// XFAIL: libc++
 
 // <functional>
 

libcxx/test/std/utilities/function.objects/func.search/func.search.bm/hash.pass.cpp

@@ -7,7 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++98, c++03, c++11, c++14
-// XFAIL: c++17, c++2a
+// XFAIL: libc++
 
 // <functional>
 

libcxx/test/std/utilities/function.objects/func.search/func.search.bm/hash.pred.pass.cpp

@@ -7,7 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++98, c++03, c++11, c++14
-// XFAIL: c++17, c++2a
+// XFAIL: libc++
 
 // <functional>
 

libcxx/test/std/utilities/function.objects/func.search/func.search.bm/pred.pass.cpp

@@ -7,7 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++98, c++03, c++11, c++14
-// XFAIL: c++17, c++2a
+// XFAIL: libc++
 
 // <functional>
 

libcxx/test/std/utilities/function.objects/func.search/func.search.bmh/default.pass.cpp

@@ -7,7 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++98, c++03, c++11, c++14
-// XFAIL: c++17, c++2a
+// XFAIL: libc++
 
 // <functional>
 

libcxx/test/std/utilities/function.objects/func.search/func.search.bmh/hash.pass.cpp

@@ -7,7 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++98, c++03, c++11, c++14
-// XFAIL: c++17, c++2a
+// XFAIL: libc++
 
 // <functional>
 

libcxx/test/std/utilities/function.objects/func.search/func.search.bmh/hash.pred.pass.cpp

@@ -7,7 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++98, c++03, c++11, c++14
-// XFAIL: c++17, c++2a
+// XFAIL: libc++
 
 // <functional>
 

libcxx/test/std/utilities/function.objects/func.search/func.search.bmh/pred.pass.cpp

@@ -7,7 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++98, c++03, c++11, c++14
-// XFAIL: c++17, c++2a
+// XFAIL: libc++
 
 // <functional>