These new options mirror -o and -output-filelist and are used instead
of those options to supply the output file path(s) to record in the
index store. This is intended to allow sharing index data across
builds in separate directories that are otherwise equivalent as far
as the index data is concered (e.g. an ASAN build and a non-ASAN build)
by supplying the same -index-unit-output-path for both.
Resolves rdar://problem/74816412
...instead of sometimes hardcoding them and sometimes using Strings.h.
The exceptions are the libraries that sit below Frontend; these can
continue using strings.
