`layout`: Use auto return type for functions that explicitly instantiate…

`layout`: Use auto return type for functions that explicitly instantiate std::tuple in return statements

This improves readability by avoiding spelling the same type twice, the first time with even more boilerplate (e.g. `typename`).
Return type deduction is a C++14 feature, and Abseil [currently supports](https://github.com/google/oss-policies-info/blob/9a9bfe8a4a12be20757497074fc2f0ecb77438ad/foundational-cxx-support-matrix.md) C++ >= 14.

PiperOrigin-RevId: 616218396
Change-Id: I82aeec878dd69001d2cf822db6512f5a62baec02

absl/container/internal/layout.h

@@ -504,9 +504,7 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>,
   // Note: We're not using ElementType alias here because it does not compile
   // under MSVC.
   template <class Char>
-  std::tuple<CopyConst<
-      Char, typename std::tuple_element<OffsetSeq, ElementTypes>::type>*...>
-  Pointers(Char* p) const {
+  auto Pointers(Char* p) const {
     return std::tuple<CopyConst<Char, ElementType<OffsetSeq>>*...>(
         Pointer<OffsetSeq>(p)...);
   }
@@ -562,9 +560,7 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>,
   // Note: We're not using ElementType alias here because it does not compile
   // under MSVC.
   template <class Char>
-  std::tuple<SliceType<CopyConst<
-      Char, typename std::tuple_element<SizeSeq, ElementTypes>::type>>...>
-  Slices(Char* p) const {
+  auto Slices(Char* p) const {
     // Workaround for https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63875 (fixed
     // in 6.1).
     (void)p;