We had ad-hoc handled some common cases by flagging sampler-typed
variables as read_only, and rejected initializers of samplers.
However, people could sneak them in in all sorts of surprising ways,
like using whole-array or structure assignment.

Fixes:
glslparsertest/glsl2/sampler-01.frag
glslparsertest/glsl2/sampler-03.frag
glslparsertest/glsl2/sampler-04.frag
glslparsertest/glsl2/sampler-06.frag

Bug #27403.