[Opaque Values] Add KeyPath support to the SIL pipeline
Enable SILGen, SIL verification, and AddressLowering of `keypath`
instructions under -enable-sil-opaque-values. Existing keypath
SIL-optimizer tests are extended to exercise the opaque-values pipeline.
In opaque-values mode, TypeLowering::handleAddressOnly lowers
address-only AST types as objects until AddressLowering runs, so
getLoweredType returns the object form for address-only KeyPath index
parameters. The keypath runtime ABI requires the address form, and
KeyPathPatternComponent caches its lowered index type — AddressLowering
does not reconstruct the pattern with refreshed lowerings. Bypass
TypeLowering in lowerKeyPathMemberIndexTypes with the static
SILType::isAddressOnly (which reads SILTypeProperties directly, skipping
handleAddressOnly's opaque-values override) and switch address-only
index lowerings to address form during pattern construction.
A cleaner fix would either drop the cached lowering from
KeyPathPatternComponent::Index, or have AddressLowering rebuild the
keypath pattern with refreshed lowerings — both larger refactors,
deferred for now.
