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.
Previously we would drop all serialized SIL from partial swiftmodule
files generated while compiling source in non-WMO mode; all that was
missing was linking it in.
This adds a frontend flag, and a test; driver change is coming up
next.
Progress on <rdar://problem/18913977>.