Make it have the semantics of subtype.

Inspired by @gnzlbg at https://github.com/rust-lang/rust/issues/46043#issuecomment-381544673

(This is just the data structure changes and some boilerplate match
code that followed from it; the actual emission of these statements
comes in a follow-up commit.)

This commit modifies the UserAssertTy statement to take a canonicalized
type rather than a regular type so that we can handle the case where the
user provided type contains a inference variable.

This is foundation for issue 46747 (limit two-phase borrows to method-call autorefs).

Sometimes a simple goto misses the cleanup/unwind edges. Specifically, in the
case of infinite loops such as those introduced by a loop statement without any
other out edges. Analogous to TerminatorKind::FalseEdges; this new terminator
kind is used when we want borrowck to consider an unwind path, but real control
flow should never actually take it.

The Abort Terminatorkind will cause an llvm.trap function call to be
emitted.
Signed-off-by: NDavid Henningsson <diwic@ubuntu.com>

FIXME: de-uglify

No functional changes intended.