Run 'AddImplicitDefineConstants' on 'BeforeCompile'

[Sergio0694]

Fixes #43908

This PR changes "AddImplicitDefineConstants" to use "BeforeCompile" as trigger, instead of "CoreCompile".

[Sergio0694]

@rainersigwald CI is all green (but I don't have permissions), can we merge this? 😄

Also, do you think we could backport this to .NET 9? Should I fill out the usual servicing form? Thank you!

[marcpopMSFT]

I'd need @rainersigwald to weigh in on how risky moving this is as we typically don't take this sort of change for 9. 9.0.2xx is open but this is borderline. We'd need high confidence of no break to consider for 1xx.

[Sergio0694]

If it helps, 9.0.2xx would also be fine (I assume that'd go out somewhere in January?). My concern was mostly about not being able to get the fix until all the way to November 2025 with .NET 10. So even just 9.0.2xx would be better than that 😅

[rainersigwald]

I think 2xx sounds reasonable for this @marcpopMSFT.

[marcpopMSFT]

Sounds reasonable. @Sergio0694 , feel free to backport to 9.0.2xx so as not to have to wait a full year.

[Sergio0694]

Awesome, will do that. Thank you both! 🙂

EDIT: opened #44938.