c++17: drop experimental::filesystem

Use std::filesystem, and drop support for building with experimental
under c++14.

Tested: Build the repo.
Change-Id: I4af0d9c034dbfef5a65153ba5447b86c961aebf1
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>
diff --git a/phosphor-ldap-mapper/main.cpp b/phosphor-ldap-mapper/main.cpp
index e0d2255..1833a6a 100644
--- a/phosphor-ldap-mapper/main.cpp
+++ b/phosphor-ldap-mapper/main.cpp
@@ -1,5 +1,5 @@
 #include <string>
-#include <experimental/filesystem>
+#include <filesystem>
 #include "config.h"
 #include "ldap_mapper_mgr.hpp"
 
@@ -13,7 +13,7 @@
                                             LDAP_MAPPER_PERSIST_PATH);
 
     // Create a directory to persist errors.
-    std::experimental::filesystem::create_directories(LDAP_MAPPER_PERSIST_PATH);
+    std::filesystem::create_directories(LDAP_MAPPER_PERSIST_PATH);
 
     // Restore the serialized LDAP group privilege mapping.
     mapperMgr.restore();