[RemoveDIs] Add flag to control loading into new debug mode from bitcode (#85649)

--load-bitcode-into-experimental-debuginfo-iterators

      false: Convert to the old debug mode after reading.
      true: Upgrade to the new debug info format (*).
      unset: Same as false (for now).

(*) As of this patch it actually just means "don't convert to either
mode after loading". Auto-upgrading will be implemented in an upcoming
patch.

With this flag we can incrementally add support for RemoveDIs by
overriding the "unset" behaviour in individual tools. The flag can be
removed once all tools support the new debug info mode.