utils: Add copy constructor to EnumSet

Some versions of clang warn as error without this:

error: definition of implicit copy constructor for 'EnumSet<tint::ast::PipelineStage>' is deprecated because it has a user-declared copy assignment operator [-Werror,-Wdeprecated-copy]
Change-Id: I02cee0b1f7b70d74f29da0067e888b50acf27ee1
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/71600
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/utils/enum_set.h

@@ -36,6 +36,10 @@ struct EnumSet {
   /// Constructor. Initializes the EnumSet with zero.
   constexpr EnumSet() = default;
 
+  /// Copy constructor.
+  /// @param s the set to copy
+  constexpr EnumSet(const EnumSet& s) = default;
+
   /// Constructor. Initializes the EnumSet with the given values.
   /// @param values the enumerator values to construct the set with
   template <typename... VALUES>