SPV: Plumb through XFB buffer and stride information.

Also, only emit this XFB information where the SPIR-V spec says
it should be emitted: essentially, on objects.

This and the previous commit together fix #1185.

Test/spv.xfb.vert

@@ -0,0 +1,20 @@
+#version 450
+
+layout(xfb_buffer = 3) out;
+layout(xfb_stride = 48) out;
+layout(xfb_offset = 12, location = 0) out float out1;
+
+layout(xfb_buffer = 2) out;
+layout(location=1) out outXfb {
+    layout(xfb_buffer = 2, xfb_stride = 32, xfb_offset = 8) float out2;
+};
+
+layout(xfb_buffer = 1, location=3) out outXfb2 {
+    layout(xfb_stride = 64, xfb_offset = 60) float out3;
+};
+
+layout(location = 4, xfb_buffer = 0, xfb_offset = 4) out float out4;
+
+void main()
+{
+}