The generic template constructor now matches the `vec<T, 4>` specialization in terms of `constexpr`.

This is only achievable with the relaxed rules on `constexpr` in C++14.

Bug: b/147359661
Change-Id: I1c63ddd1fbdf3c491f8fcdba96047be0665f122f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/40209Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>