SPV: Allow push constant buffers to not have an instance name.

Test/spv.pushConstantAnon.vert

@@ -0,0 +1,17 @@
+#version 400
+
+layout(push_constant) uniform Material {
+    int kind;
+    float fa[3];
+};
+
+out vec4 color;
+
+void main()
+{
+    switch (kind) {
+    case 1:  color = vec4(0.2); break;
+    case 2:  color = vec4(0.5); break;
+    default: color = vec4(fa[1]); break;
+    }
+}