Generate Abort instead of Resume terminators on nounwind ABIs.

https://github.com/rust-lang/rust/issues/18510Signed-off-by: NDavid Henningsson <diwic@ubuntu.com>

This simplifies analysis and borrow-checking because liveness at the
resume point can always be simply propagated.

Later on, the "dead" Resumes are removed.

Improves #44832

borrowck-overloaded-index-move-index.rs - fixed
borrowck-multiple-captures.rs - still ICE
borrowck-issue-2657-1.rs - fixed
borrowck-loan-blocks-move.rs - fixed
borrowck-move-from-subpath-of-borrowed-path.rs - fixed
borrowck-mut-borrow-linear-errors.rs - still ICE
borrowck-no-cycle-in-exchange-heap.rs - fixed
borrowck-unary-move.rs - fixed
borrowck-loan-blocks-move-cc.rs - fixed
borrowck-vec-pattern-element-loan.rs - still broken

The previous code would leak data on a drop panic if the immediate next
drop was a StorageDead that was followed by another drop.

Fixes #45328.

No functional changes intended.

(The idea is that the StorageDead marks the point where the memory can
be deallocated, and the EndRegion is marking where borrows of that
memory can no longer legally exist.)

The main intent is to fix cases where EndRegion emission is believed
to be causing excess peak memory pressure.

It may also be a welcome change to people inspecting the MIR output
who find the EndRegions to be a distraction.

Small cleanup that shouldn't have any impact

Like #43008 (f6689991), but _much more aggressive_.

This falls naturally out of making drop elaboration work with `box`
expressions, which is probably required for sane MIR borrow-checking.
This is a pure refactoring with no intentional functional effects.

The unused blocks are removed by SimplifyCfg, but they can cause a
significant performance slowdown before they are removed.