[m-rev.] for review: make foreign_procs a top-level item

[Zoltan Somogyi]

For review by anyone.

This diff caused two tests to fail. The diff to make_hlds_passes.m contains
a ZZZ comment documenting the reason for this, and outlines a solution:
adding marker pragmas to the HLDS separately from other kinds of
impl pragmas. Does anyone object to this solution? Does anyone object
to doing it by making marker pragmas their own kind of pragma, separate
from decl, impl and gen pragmas? I would like feedback on this in the next
day or so, since I want to do that next.

That comment also outlines a longer term proposal: deprecating promise
pure/semipure pragmas. I would also like feedback on that, but this question
is not that urgent.

Zoltan.
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