Apart from mildly speeding up indexing, this also keeps the compiler
from running into issues with implementation-only imports that may not
be present while we're trying to index.
rdar://problem/52083709
We previously shied away from this in order to not /accidentally/
depend on it, but it becomes interesting again with textual
interfaces, which can certainly be read by humans. The cross-file
order is the order of input files, which is at least controllable by
users.
This converts the instances of the pattern for which we have a proper
substitution in lit. This will make it easier to replace it
appropriately with Windows equivalents.
