GLSL: Implement XFB for redeclared built-in blocks.

Test/spv.builtInXFB.vert

@@ -0,0 +1,15 @@
+#version 450
+
+layout(xfb_buffer = 1, xfb_stride = 64) out;
+
+layout (xfb_buffer = 1, xfb_offset = 16) out gl_PerVertex
+{
+    float gl_PointSize;
+    vec4 gl_Position;
+};
+
+void main()
+{
+    gl_Position = vec4(1.0);
+    gl_PointSize = 2.0;
+}