We ultimately want to explicitly change standard library uses of #file to #fileID, but once we do, previous compilers won’t be able to build the standard library. So instead, we will temporarily build the standard library with -enable-experimental-concise-pound-file, which should have the same effect, but will back-deploy to compilers going back several months.
Add -experimental-enable-concise-pound-file to the list of flags preserved by module interfaces, so that when we rebuild an interface, it comes out the same way as the original file.
In -swift-version 5 and earlier, #file will continue to be a synonym for #filePath; in a future -swift-version (“Swift 6 mode”), it will become a synonym for #fileID. #file in libraries will be interpreted according to the language mode the library was compiled in, not the language mode its client uses.
Implement this behavior, tied to a frontend flag instead of a language version. We do so by splitting the old `MagicIdentifierLiteralExprKind::File` into two separate cases, `FileIDSpelledAsFile` and `FilePathSpelledAsFile`, and propagating this distinction throughout the AST. This seems cleaner than looking up the setting for the module the declaration belongs to every time we see `File`.
This doesn’t handle module interfaces yet; we’ll take care of those in a separate commit.
