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.
It is invalid if the same decoration is applied to the same id multiple
times. This adds a check before adding a decoration that the decoration
is not already in the list. If it is, then the duplicate is not added.
Fixes#3627