Change binding auto-map to use provided offsets.

Previously, the binding auto-mapping facility was free to use any unused
binding.  This change makes auto-bindings use the same offset value as
explicit bindings.

Test/baseResults/spv.buffer.autoassign.frag.out

@@ -35,11 +35,11 @@ Linked fragment stage:
                               MemberDecorate 25 0 Offset 0
                               Decorate 25 Block
                               Decorate 27 DescriptorSet 0
-                              Decorate 27 Binding 1
+                              Decorate 27 Binding 15
                               MemberDecorate 31 0 Offset 0
                               Decorate 31 Block
                               Decorate 33 DescriptorSet 0
-                              Decorate 33 Binding 2
+                              Decorate 33 Binding 16
                               Decorate 41(Color) Location 0
                2:             TypeVoid
                3:             TypeFunction 2