[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/module.modulemap b/include/module.modulemap
index b6a39a0..783f212 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -789,9 +789,9 @@
     export *
 
     module __utility {
-      module __decay_copy        { private header "__utility/__decay_copy.h"        }
       module as_const            { private header "__utility/as_const.h"            }
       module cmp                 { private header "__utility/cmp.h"                 }
+      module decay_copy          { private header "__utility/decay_copy.h"          }
       module declval             { private header "__utility/declval.h"             }
       module exchange            { private header "__utility/exchange.h"            }
       module forward             { private header "__utility/forward.h"             }