System/Types.hpp: Make vec(T replicate) constexpr

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/+/40209
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
diff --git a/src/System/Types.hpp b/src/System/Types.hpp
index d99205b..165df56 100644
--- a/src/System/Types.hpp
+++ b/src/System/Types.hpp
@@ -60,7 +60,7 @@
 {
 	vec() = default;
 
-	explicit vec(T replicate)
+	constexpr explicit vec(T replicate)
 	{
 		for(int i = 0; i < N; i++)
 		{