Remove conditional stopping creating of constraint variables

[john-h-kastner]

This fixes #297 which is blocking libarchive. It's a simple fix, but I'm a little hesitant to remove the if (FE && FE->isValid()) conditional without understanding why it was there in the first place.

[mattmccutchen-cci]

I guess the condition must be false in the example of #297 in order for your change to make a difference? Can you print out some more information (or attach the debugger) and find out more about the significance of it being false (the file entry not being valid) in this case? (Maybe I'm stating the obvious...)

Duh, I read #297 now. The condition is false because the struct c is inside a macro. So your question is why we thought it was unsafe to add a constraint variable when the struct c is inside a macro. I don't know, but in general, we don't have a good way to control for bugs that don't show up in any of our tests, so I'd say go for it.