Also fixes, in practice, https://github.com/KhronosGroup/GLSL/issues/83.
When the specification language is correctly created, glslang can be
revisited for correctness.  In the meantime, this seems like the best
"bug" to have relative to the specification.

Memory qualifiers are only relevant to parameters when they apply
to what the argument points to, as otherwise the argument is copied.

This leaves the fix from #1870 in place, and then more correctly
ignores memory qualifiers when something will be passed by copy.