[ORC] Support scanning "fallback" slices for interfaces. (#168472)
When scanning an interface source (dylib or TBD file), consider
"fallback" architectures (CPUType / CPUSubType pairs) in addition to the
process's CPUType / CPUSubType.
Background:
When dyld loads a dylib into a process it may load dylib or slice whose
CPU type / subtype isn't an exact match for the process's CPU type /
subtype. E.g. arm64 processes can load arm64e dylibs / slices.
When building an interface we need to follow the same logic, otherwise
we risk generating a spurious "does not contain a compatible slice"
error. E.g. If we're running an arm64 JIT'd program and loading an
interface from a TBD file, and if no arm64 slice is present in that
file, then we should fall back to looking for an arm64e slice.
rdar://164510783