CAS support in compiler relies on supplementary paths to decide the mapping between input and output files. Therefore, we
have to compute the paths of the module ObjC trace files in this canonical place to have CAS support for
this newly added ObjC message trace files.
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
Follow programming guidelines for these getters more closely and have them return a non-owning view of the underlying data instead of relying on callers to take const references to the copy that is returned here.
These inputs were previously modeled as Swift files, which would lead to bizarre situations where parts of the pipeline expecting Swift inputs actually wound up parsing Objective-C.
This means moving the output path into SupplementaryOutputPaths, and
using the same sort of diagnostic dispatching that serialized
diagnostics use. This is part of what's needed to run the migrator
in batch mode.
