Replace absl::IsConvertibleHelper with std::is_convertible

PiperOrigin-RevId: 495898835
Change-Id: Id2150a0505020ac2be4487e904d167a26316260c

absl/types/internal/span.h

@@ -99,28 +99,9 @@ bool LessThanImpl(SpanT<T> a, SpanT<T> b) {
   return std::lexicographical_compare(a.begin(), a.end(), b.begin(), b.end());
 }
 
-// The `IsConvertible` classes here are needed because of the
-// `std::is_convertible` bug in libcxx when compiled with GCC. This build
-// configuration is used by Android NDK toolchain. Reference link:
-// https://bugs.llvm.org/show_bug.cgi?id=27538.
-template <typename From, typename To>
-struct IsConvertibleHelper {
- private:
-  static std::true_type testval(To);
-  static std::false_type testval(...);
-
- public:
-  using type = decltype(testval(std::declval<From>()));
-};
-
-template <typename From, typename To>
-struct IsConvertible : IsConvertibleHelper<From, To>::type {};
-
-// TODO(zhangxy): replace `IsConvertible` with `std::is_convertible` once the
-// older version of libcxx is not supported.
 template <typename From, typename To>
 using EnableIfConvertibleTo =
-    typename std::enable_if<IsConvertible<From, To>::value>::type;
+    typename std::enable_if<std::is_convertible<From, To>::value>::type;
 
 // IsView is true for types where the return type of .data() is the same for
 // mutable and const instances. This isn't foolproof, but it's only used to