Apply GLSL memory decorations to top-level OpVariable

Apply memory decorations from GLSL source to the top-level OpVariable.
Previously, these decorations would only be applied to individual
members. While this is correct behavior, it is more convenient for some
front ends to see the decorations (specifically ReadOnly and WriteOnly)
applied to the whole variable rather than individual members.

Test/baseResults/spv.memoryQualifier.frag.out

@@ -65,6 +65,7 @@ Validation failed
                               MemberDecorate 50(Buffer) 4 Offset 48
                               MemberDecorate 50(Buffer) 5 Coherent
                               MemberDecorate 50(Buffer) 5 Offset 56
+                              Decorate 52 Coherent
                               Decorate 52 Binding 4
                               Decorate 52 DescriptorSet 0
                2:             TypeVoid