[gtest] Use std::index_sequence and friends instead of rolling our own

* Applies for `std::index_sequence`, `std::make_index_sequence`, and `std::index_sequence_for` replacing `IndexSequence`, `MakeIndexSequence` and IndexSequenceFor`
* Also deleted implementation helper `DoubleSequence`
* The standard interfaces [have been in the standard library since C++14](https://en.cppreference.com/w/cpp/utility/integer_sequence), which [is the minimum supported C++ version by Google Test](https://github.com/google/oss-policies-info/blob/main/foundational-cxx-support-matrix.md)

PiperOrigin-RevId: 621939977
Change-Id: Id264266f08da66c0fa2a6e6fbb8f86fd3cb3a421

googlemock/src/gmock-internal-utils.cc

@@ -44,6 +44,7 @@
 #include <iostream>
 #include <ostream>  // NOLINT
 #include <string>
+#include <utility>
 #include <vector>
 
 #include "gmock/gmock.h"
@@ -211,14 +212,14 @@ constexpr char UnBase64Impl(char c, const char* const base64, char carry) {
 }
 
 template <size_t... I>
-constexpr std::array<char, 256> UnBase64Impl(IndexSequence<I...>,
+constexpr std::array<char, 256> UnBase64Impl(std::index_sequence<I...>,
                                              const char* const base64) {
   return {
       {UnBase64Impl(UndoWebSafeEncoding(static_cast<char>(I)), base64, 0)...}};
 }
 
 constexpr std::array<char, 256> UnBase64(const char* const base64) {
-  return UnBase64Impl(MakeIndexSequence<256>{}, base64);
+  return UnBase64Impl(std::make_index_sequence<256>{}, base64);
 }
 
 static constexpr char kBase64[] =