[SYCL] Make mutable swizzle functions unusable on const vec

[steffenlarsen]

Swizzles should not expose mutating functions when the underlying vector
is const. This commit SFINAEs these out.

This is built on top of #13012.

[gmlueck]

I didn't really review the code closely, but I ran some test cases. Everything looked good, except this one which won't compile:

const sycl::vec<int, 4> v4{0};
using type = decltype(v4.swizzle<0>())::value_type;

It seems like the __swizzled_vec__ type does not define the value_type member type. I think this should be an alias to the same type as element_type (which is int in this case).

[steffenlarsen]

I didn't really review the code closely, but I ran some test cases. Everything looked good, except this one which won't compile:

const sycl::vec<int, 4> v4{0};
using type = decltype(v4.swizzle<0>())::value_type;

It seems like the __swizzled_vec__ type does not define the value_type member type. I think this should be an alias to the same type as element_type (which is int in this case).

Good find! Adding those can be done in separation, as I have done here: #13040

[steffenlarsen]

KhronosGroup/SYCL-Docs#514 has been merged. This is ready for review! Keep in mind that it is based on #13012.

[steffenlarsen]

Discussing this with Andrei, we agreed that the proposed variant was too intrusive on the signatures compared to the new spec definitions. Instead, I have moved the operators to new base-classes, which are in turn specialized to match the requirements of the various operators.

Note: I included the changes as part of the merge commit by accident.