Fix is_blittable partial specializations (dotnet/coreclr#6357)

Fix is_blittable partial specializations (dotnet/coreclr#6357)

The is_blittable partial specializations were defined using "class", privately
inheriting from std::true_type. This means the ::value member variable will
be inaccessible to most users of these types. Thus the type
``std::enable_if<is_blittable<T>::value>::type'' will always result in a
substitution failure with a compiler that respects accessibility in SFINAE.

This commit changes "class" to "struct" for these partial specializations
so they inherit publicly from std::true_type.

Commit migrated from https://github.com/dotnet/coreclr/commit/8cdcdf1e8e2ed789c8a689553a384bb6fd1d7bcd