Fix bug #183

--warn-dead-procs should not emit warnings for procedures that are foreign
exported.  However, for procedures that were foreign exported only to languages
that were *not* supported by the current backend, it did emit warnings.
This was because we only kept track of foreign_export pragmas for languages
that were supported by the current backend.  The fix is to (separately) keep
track of whether there were any foreign_export pragmas at all for a procedure.

compiler/hlds_pred.m:
	Add a new field to the proc_info structure that keeps track
	of whether the were any foreign_export pragmas for the procedure.

compiler/add_pragma.m:
	Fill in the new field.

compiler/dead_proc_elim.m:
	Only emit a warning about a procedure being dead if it is not
	the subject of any foreign_export pragmas.

	Use a more appropriate variable name in a spot.

tests/valid/Mmakefile:
tests/valid/bug183.m:
	Add a regression test for the above.