SCons: Set silence_msvc to false by default

[Repiteo]

Because MSVC sometimes outputs errors onto stdout instead of stderr for some ungodly reason, the silence_msvc option isn't the surefire workaround I first thought. Left the functionality as-is for now, but changed the description to warn about potentially hiding error messages & made the default setting false.

[Calinou]

Can we detect if stdout line printed by MSVC is of form path/to/file.something (with nothing else on the line), and only silence it in that case? I think this would be preferable to making #87154 ineffective by default.

[Repiteo]

I imagine that's possible, but I'm unsure how to do that atm; this is very much a short-term solution PR

[akien-mga]

Thanks!