Depend on rustc_abi in compiler crates that use it indirectly but have
not yet taken on that dependency, and are not entangled in my other PRs.
This leaves an "excise rustc_target" step after the dust settles.
The pattern-analysis code needs to print patterns, as part of its user-visible
diagnostics. But it never actually tries to print "real" patterns! Instead, it
only ever prints synthetic patterns that it has reconstructed from its own
internal represenations.
We can therefore simultaneously remove two obstacles to changing `thir::Pat`,
by having the pattern-analysis code use its own dedicated type for building
printable patterns, and then making `thir::Pat` not printable at all.
