Implement the infrastructure for feature-test macros. Very few actual feature test macros, though. Reviewed as: https://reviews.llvm.org/D51955

llvm-svn: 342073
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: f56972e22489c92028e5147344ab8b31b0b4fed5
diff --git a/include/filesystem b/include/filesystem
index aa1d718..7547e6c 100644
--- a/include/filesystem
+++ b/include/filesystem
@@ -244,6 +244,7 @@
 #include <utility>
 #include <iomanip> // for quoted
 #include <string_view>
+#include <version>
 
 #include <__debug>
 
@@ -256,10 +257,6 @@
 
 #ifndef _LIBCPP_CXX03_LANG
 
-#if _LIBCPP_STD_VER >= 17
-#define __cpp_lib_filesystem 201703
-#endif
-
 _LIBCPP_BEGIN_NAMESPACE_FILESYSTEM
 
 struct _FilesystemClock {