#4 semantic analysis for multi foreign_proc

[Tyson Dowd]

> Ideally we ought to perform semantic analysis on
Mercury clauses
> even if they are overridden by foreign language
pragmas.
> Your patch doesn't do that. However, that can
remain a wishlist item
> for some future patch...

Yes, this would be nice, as would error checking (e.g.
unused variable
name checking) the foreign_procs even if we aren't
using them.

(from July 30 tread on mercury-reviews, not yet
available as a URL).