`build_session` is passed an `EarlyErrorHandler` and then constructs a
`Handler`. But the `EarlyErrorHandler` is still used for some time after
that.
This commit changes `build_session` so it consumes the passed
`EarlyErrorHandler`, and also drops it as soon as the `Handler` is
built. As a result, `parse_cfg` and `parse_check_cfg` now take a
`Handler` instead of an `EarlyErrorHandler`.
apparently I missed some tests in the last commit. Rather than having
dozens of tests use the long version, use the short version in
`run-make` and the long version in `run-make-fulldeps` (which is now
only three tests)
