Ignore unexpected incr-comp session dirs
Clearly the code path can be hit without the presence of a compiler bug.
All it takes is mischief. See #71698.
Ignore problematic directories instead of ICE:ing. `continue`ing is
already done for problematic dirs in the code block above us.
Closes#71698.
With this fix, the output is this instead of ICE:
```
$ cargo +stage1 new gz-ice && cd gz-ice
$ cargo +stage1 build
$ find target -type f -exec gzip {} \;
$ cargo +stage1 run
Created binary (application) `gz-ice` package
Compiling gz-ice v0.1.0 (/tmp/gz-ice)
Finished dev [unoptimized + debuginfo] target(s) in 0.13s
gzip: target/debug/gz-ice has 1 other link -- unchanged
gzip: target/debug/deps/gz_ice-de919414dd9926b9 has 1 other link -- unchanged
Compiling gz-ice v0.1.0 (/tmp/gz-ice)
warning: failed to garbage collect invalid incremental compilation session directory `/tmp/gz-ice/target/debug/incremental/gz_ice-23qx9z9j9vghe/s-gnwd8daity-kp10sj.lock.gz`: Not a directory (os error 20)
warning: `gz-ice` (bin "gz-ice") generated 1 warning
Finished dev [unoptimized + debuginfo] target(s) in 0.13s
Running `target/debug/gz-ice`
Hello, world!
```
Matthias Krüger
e25dfe1ef6