swift234

[cmpcodesize] Assert env vars set properly

`$SWIFT_OLD_BUILDDIR` and `$SWIFT_NEW_BUILDDIR` are only necessary when
not using `--` to delineate two sets of files.

1. Only grab them from the environment when `--` is *not* used.
2. Assert if they're not present in that codepath. Print what *is*
   specified in the environment, in order to help the user correct
   their mistake.