Generator expression handled incorrectly when there is if

[TotalVerb]

julia> lintstr("""
       f(x) = [tag for tag in x if iseven(tag)]
       """)
2-element Array{Lint.LintMessage,1}:
 none:1 E321 tag: use of undeclared symbol    
 none:1 I382 x: argument declared but not used

[HarrisonGrodin]

This seems to show up across the board with comprehensions.

julia> lintstr("""
       f() = [x for x = 1:10 if iseven(x)]
       g() = (y for y = 1:10 if iseven(x))
       h() = Dict(z => z^2 for z = 1:10 if iseven(z))
       """)
6-element Array{Lint.LintMessage,1}:
 none:1 E321 x: use of undeclared symbol
 none:2 E321 y: use of undeclared symbol
 none:3 E321 z: use of undeclared symbol
 none:3 E321 z: use of undeclared symbol
 none:3 E321 z: use of undeclared symbol
 none:3 E321 z: use of undeclared symbol

[TotalVerb]

Yep, luckily I think all these cases can be handled similarly. I'll have a fix this weekend.