How to reproduce:
1. Open the attached "ScriptBreakingUnity" project
2. In the Project window select the "breakit" script
3. Open the Inspector (Window->General->Inspector)

Expected results: There's no "No MonoBehaviour scripts in the file, or their names do not match the file name." message
Actual results: There's a "No MonoBehaviour scripts in the file, or their names do not match the file name." message

Reproducible with: 2020.1.0a7, 2020.1.6f1, 2020.2.0b3
Not reproducible with: 2018.4.27f1, 2019.4.11f1, 2020.1.0a6

Notes:
- 2020.1.0a7 is the first version where the issue appears when creating a new project (not upgrading/downgrading)
- Upgrading a project with the script up to Unity 2020.1.0a14 will not reproduce the issue if it wasn't there initially
- When downgrading a project that has the issue to Unity 2020.1.0a2 or later will keep the issue around but resetting packages sometimes fixes it
- If the #if block with the grave accent is not inside a namespace Unity will not throw any errors and will recognise the script as MonoBehaviour
- If the #if block is set to true, the Console will throw the "Unexpected character" error and the Editor will accept the script as MonoBehaviour script