In ecpg preprocessor, don't try to look up constants in the test for

In ecpg preprocessor, don't try to look up constants in the test for
variable hiding. A constant is not a variable. It worked in most cases by
accident, because we add constants to the global list of variables (why?),
but float constants like 1.23 were interpreted as struct field references,
and not found.

Backpatch to 9.0, where the test for variable hiding was added.