Declarations of type sampler_t are now checked to be in the private or constant address space.

This allows to introduce global-scope sampler_t constants. Such
constants may be placed to a constant address space struct, but it's
ok, because they cannot be mutated anyway through that.
WebCLAnalyser:handleVarDecl: global scope private variables are now collected

Added also simple test case variable-global-scope.el.