Using the GPULM with baked lights and when trying to cast a shadow via a backface results in weird artifacts close to the occluder.
The CPULM doesn't have this issue. (See snapshots below)

Steps to reproduce:

1. Download and open the attached project (backface-shadow.zip)
2. Open SampleScene
3. In the Lighting window, make sure that Progressive GPU is set as the lightmapping backend
4. Generate lighting
5. Observe the artifacts on the ground plane

Notes:

- Reproduces in: 2019.4.21f1 (b76dac84db26), 2020.1.0a11 (ec790d93e91f), 2020.1.0a15 (c34b21e125d7), 2020.1.0a16 (196f78887a11)
- Does not reproduce in: 2020.1.0a17 (ccc6f9131a63), 2020.1.0a20 (019325219793), 2020.1.0b1 (92a30bddf81d), 2020.1.17f1 (9957aee8edc2), 2020.2.5f1 (e2c53f129de5), 2021.1.0b4 (6d90c0720bd4), 2021.2.0a5 (4a734693b889), 2021.2.0a6 (dc497bf8326a), 2021.2.0a7 (1caa39c8ddb6), 2021.2.0a8 (05e6d22b19f7)
- Affects GPU PLM only