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.
