Investigate `-prefix-serialized-debugging-options`

[keith]

As of Swift 5.6(?) there is a new -prefix-serialized-debugging-options flag which is supposed to apply -debug-prefix-map flags to the serialized debugging options that we disable with the swift.cacheable_swiftmodules feature. Theoretically if we can switch to this flag, we can stop disabling these which might reduce the amount of work that has to be done in user's lldbinit files.