SCons: Silence MSVC output bloat without hiding errors

[Repiteo]

Thanks to the SCons SPAWN feature, we can effectively catch stdout before it even calls to actions in the first place. This functionality is now more specialized to only care about cl/link calls, otherwise opting for the normal SPAWN with no changes. If the captured content is a single line, it's bloat & gets hidden. Multiple lines means it captured an error, and will redirect the content to stderr. As the behavior is now working as originally intended, #89159 was reverted to make silence_msvc enabled by default once more.

[Calinou]

Tested locally, it works as expected (Windows 11 23H2, MSVC 2022). I also checked error messages still appear when they should by introducing an error and compiling.

A small nitpick is that the filename is still displayed on its own line when there's an error:

scons: Building targets ...
Compiling main\main.cpp ...
main.cpp
main\main.cpp(31): error C2006: '#include': expected "FILENAME" or <FILENAME>
main\main.cpp(31): fatal error C1083: Cannot open include file: '': No such file or directory
scons: *** [main\main.windows.editor.x86_64.obj] Error 2
scons: building terminated because of errors.

It could be interesting to strip the lone main.cpp from the error message, but it's not critical.

[Repiteo]

Somehow I didn't notice that the initial line of bloat was always there. Thankfully, this simplifies the logic a fair bit, making the stderr call a one-liner.

[akien-mga]

Thanks!