Expose template parameters of endian specific types as class members

[labath]

Summary

This allows generic code to query these parameters, and is a common
practice in a lot of other template classes. I'm mainly interested in
the ValueType paramemter here, but I've added others for symmetry.

[Bigcheese]

What's the use case?

[labath]

The use case is being able to convert from a ulittle32_t (and friends) into an enum class. Since both are strongly typed, I cannot convert between one and the other without an intermediate cast to the basic type (and so, in generic code, I need to know what that type is). You can see it in action in D58976 (just search for value_type).

[Bigcheese]

Ah, I see. This seems fine.

include/llvm/Support/Endian.h

memcpy(LLVM_ASSUME_ALIGNED(
            &val[0], sizeof(value_type) * 2);
   }
 }
 
 } // end namespace endian
 
 namespace detail {
 
-template<typename value_type,
-         endianness endian,
-         std::size_t alignment>
+template<typename ValueType,
+         endianness Endian,
+         std::size_t Alignment>
 struct packed_endian_specific_integral {
+  using value_type = ValueType;
+  static constexpr endianness endian = Endian;
+  static constexpr std::size_t alignment = Alignment;
+
   packed_endian_specific_integral() = default;
 
   explicit packed_endian_specific_integral(value_type val) { *this = val; }
 
   operator value_type() const {
     return endian::read<value_type, endian, alignment>(
       (const void*)Value.buffer);
   }