Convert some type trait backfills into aliases to reduce template instantiations

PiperOrigin-RevId: 1008369136

absl/meta/type_traits.h

diff --git a/absl/meta/type_traits.h b/absl/meta/type_traits.h
index 51dfb27..9567533 100644
--- a/absl/meta/type_traits.h
+++ b/absl/meta/type_traits.h
@@ -245,9 +245,7 @@
 // C++17 compatible implementation of std::remove_cvref which was added in
 // C++20.
 template <typename T>
-struct remove_cvref {
-  using type = std::remove_cv_t<std::remove_reference_t<T>>;
-};
+using remove_cvref = std::remove_cv<std::remove_reference_t<T>>;
 
 template <typename T>
 using remove_cvref_t = typename remove_cvref<T>::type;
@@ -264,9 +262,7 @@
 //
 // Back-fill of C++20's `std::type_identity`.
 template <typename T>
-struct type_identity {
-  typedef T type;
-};
+using type_identity = std::enable_if<true, T>;
 
 // type_identity_t
 //