SPV: Nested layouts: Recursively send around the top-level std140 or std430 packing.

This also distinguishes between the same struct included in multiple parent packings.

Test/spv.layoutNested.vert

@@ -0,0 +1,31 @@
+#version 450
+
+// should get 3 SPV types: no layout, 140, and 430
+struct S
+{
+	highp uvec3 a;
+	mediump mat2 b[4];
+	lowp uint c;
+};
+
+layout(set = 0, binding = 0, std140) uniform Block140
+{
+	mediump int u;
+	S s[2][3];
+	mediump vec2 v;
+} inst140;
+
+layout(set = 0, binding = 1, std430) buffer Block430
+{
+	mediump int u;
+	S s[2][3];
+	mediump vec2 v;
+} inst430;
+
+S s;
+
+void main()
+{
+    s.c = inst140.u;
+    gl_Position = vec4(s.c);
+}