HLSL: Fix #1154: Support PointSize, as an attribute.

Test/hlsl.PointSize.geom

@@ -0,0 +1,11 @@
+struct S {
+    [[vk::builtin("PointSize")]] float ps;
+};
+
+[maxvertexcount(4)]
+void main([[vk::builtin("PointSize")]] triangle in uint ps[3],
+       inout LineStream<S> OutputStream)
+{
+    S s;
+    OutputStream.Append(s);
+}