[hl] add -D hl_w_null_compare

[yuxiaomao]

I'm trying to add a new flag -D hl_w_null_compare which will gives a warning where we have done null comparison instead of implicitly convert to 0. It can help existing hl code check theirs comparison before moving to the newer version. Related commits:

• [hl] Rework Null<Int/Float/Bool> comparison for spec/alloc #11612
• [hl] Fix NotEq compare nullInt with Int #11629

This code works correctly during the build, however, when running inside VSCode and open the file from the "PROBLEMS" section after build, the warning disappear after a small delay (probably some rebuild on the specific file happens). It does not impact all files, at least not std files.
@kLabz du you have any idea on how to prevent the warning been removed?

[kLabz]

Other than disabling diagnostics, probably no way atm since the warning comes from generator so won't happen in diagnostics :/

[yuxiaomao]

Thanks for pointing out the diagnostics feature! I did not know it before. My PR is only a tool for upgrading new haxe version, disable temporary diagnostics is totally acceptable. So for now I have this:
"haxe.enableDiagnostics": false in settings.json
-D hl_w_null_compare in build.hxml

I'll check with the dev team to see if this fit theirs need

[yuxiaomao]

No one says no, so I'll merge it!

[kLabz]

Not sure we'll want to keep it as is for long, since warnings in generators should be avoided as much as possible :/

[yuxiaomao]

I think it's an interesting tools for other hl project pass to haxe 5.0 - It can be a warning before generator but I don't think other target are concerned in this specific case?
Note: This commit can be revert safely.