Fix missing entries in upvar borrows map for capture-by-ref unboxed closures

This prevents a later ICE in borrowck.

Closes issue #17655
Fix categorization of upvars of capture-by-reference unboxed closures

In particular, this causes mutation of an upvar to correctly mark
it as mutable during adjustment.  This makes borrowck correctly
flag conflicting borrows, etc.

We still seem to generate incorrect code in trans which copies the upvar
by value into the closure.  This remains to be fixed.
Return correct types for capture-by-ref unboxed closure upvars

Treat upvars of capture-by-reference unboxed closures as references
with appropriate regions and mutability.
Correctly trans capture-by-ref unboxed closures

Store references to the freevars instead of copies when constructing
the environment and insert an additional load when reading them from
the environment.
Rehabilitate an unboxed closure test

This test works as a regression test for issue #17655.  It also
exercises mutation of by-ref upvars.
Add some more test coverage of by-ref unboxed closures
auto merge of #17731 : bkoropoff/rust/unboxed-by-ref, r=pcwalton

This began as an attempt to fix an ICE in borrowck (issue #17655), but the rabbit hole went pretty deep.  I ended up plumbing support for capture-by-reference unboxed closures all the way into trans.

Closes issue #17655.