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
This adds two things:
- Calling `swiftc -dump-ast foo.swift [...] -o foo.ast` will dump the AST to the file `foo.ast`, instead of dumping it to `stderr` as usual.
- Calling `swiftc -dump-ast -output-file-map=outputFileMap.json *.swift [...]`, given an `outputFileMap.json` file that contains entries in the form `"ast-dump": "foo.ast"`, will dump the ASTs of the input files to their respective output files in the file map.
This should serve as a valid workaround to a bug mentioned in [the forums](https://forums.swift.org/t/error-when-dumping-the-ast-for-hundreds-of-files/17578) where the AST dump functionality crashes when called with too many input files. A few implementation details were also discussed in the same forum post.
As an aside, this also fixes a comment in `include/swift/Basic/PrimarySpecificPaths.h` that was incorrect.
