Rollup merge of #140120 - ChrisDenton:mir-opt-dump-rev, r=jieyouxu

Use `output_base_dir` for `mir_dump_dir`

It just occurred to me that the problem might be due to multiple revisions using the same dump directory (and therefore deleting the other revision's dir). This fixes that by simply using the normal per-test output directory, which is revision safe.

Author: ChrisDenton