[libc++][NFC] Fix include guard for decay_copy.h and remove underscores from the header
We don't use double underscores for private header names when they are
in a subdirectory with double underscores already.
Differential Revision: https://reviews.llvm.org/D108820
NOKEYCHECK=True
GitOrigin-RevId: c8b14c03ec741f672f0ca1370ecdd9415d7eca7c
diff --git a/include/future b/include/future
index 349e6af..89acdba 100644
--- a/include/future
+++ b/include/future
@@ -366,7 +366,7 @@
#include <__debug>
#include <__memory/allocator_arg_t.h>
#include <__memory/uses_allocator.h>
-#include <__utility/__decay_copy.h>
+#include <__utility/decay_copy.h>
#include <__utility/forward.h>
#include <chrono>
#include <exception>