Previously, we'd combine just the target architecture, and rely on the
fact that the .swiftinterface is in a reasonably-target-specific
subdirectory to include enough entropy to avoid hash collisions. But in
the presence of a VFS or if two targets are sharing the same
.swiftinterface file (which can sometimes happen in tests), they will
collide since the hash only includes architecture.
Instead, use the same normalization that the serialized module loader
uses, and serialize the normalized target triple instead.
Fixes rdar://55881335
