Avoid redundant calls to filesystem::status_known

	* include/bits/fs_ops.h (exists(const path&, error_code&))): Only
	check status_known once.
	* include/experimental/bits/fs_ops.h: Likewise.

From-SVN: r256243

libstdc++-v3/ChangeLog

 2018-01-04  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/bits/fs_ops.h (exists(const path&, error_code&))): Only
+	check status_known once.
+	* include/experimental/bits/fs_ops.h: Likewise.
+
 	PR libstdc++/83607
 	* include/std/functional (__is_byte_like): New trait.
 	(__is_std_equal_to): Remove.

libstdc++-v3/include/bits/fs_ops.h

@@ -125,8 +125,11 @@ namespace filesystem
   {
     auto __s = status(__p, __ec);
     if (status_known(__s))
-      __ec.clear();
-    return exists(__s);
+      {
+	__ec.clear();
+	return __s.type() != file_type::not_found;
+      }
+    return false;
   }
 
   uintmax_t file_size(const path& __p);

libstdc++-v3/include/experimental/bits/fs_ops.h

@@ -131,8 +131,11 @@ inline namespace v1
   {
     auto __s = status(__p, __ec);
     if (status_known(__s))
-      __ec.clear();
-    return exists(__s);
+      {
+	__ec.clear();
+	return __s.type() != file_type::not_found;
+      }
+    return false;
   }
 
   uintmax_t file_size(const path& __p);