Allow a `std::move` of `delimiter_` to happen in `ByString::ByString(ByString&&)`. Right now the move ctor is making a copy because the source object is `const`.

PiperOrigin-RevId: 778791853
Change-Id: Ia1ef1a0c525aebc03509547b81c4f6e005971fa6

absl/strings/str_split.h

diff --git a/absl/strings/str_split.h b/absl/strings/str_split.h
index 761568a..cf53ccf 100644
--- a/absl/strings/str_split.h
+++ b/absl/strings/str_split.h
@@ -127,7 +127,7 @@
   absl::string_view Find(absl::string_view text, size_t pos) const;
 
  private:
-  const std::string delimiter_;
+  std::string delimiter_;
 };
 
 // ByAsciiWhitespace