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No point in pretending other compilers can use the GHC testsuite. This
makes the *.T files a bit shorter.

Previously we were capturing the *entire environment* when moving under
a 'proc', for the newArrowScope/escapeArrowScope thing.  But that a blunderbuss,
and in any case isn't right (the untouchable-type-varaible invariant gets
invalidated).

So I fixed it to be much more refined: just the LocalRdrEnv and constraints are
captured.

I think this is right; but if not we should just add more fields to ArrowCtxt,
not return to the blunderbuss.

This patch fixes the ASSERT failure in Trac #5267

arrowfail001 currently fails only when DEBUG is defined.

Rearrange arrows tests, and move tc120fail to arrowfail003.