FSC: Consider enabling `--parallelreferenceresolution` by default

[safesparrow]

#13521 introduces an FSC flag --parallelreferenceresolution that makes dll imports run in parallel.
I ran some tests and it gave 5% relative/600ms absolute speedup of single-project dotnet build for projects with lots of NuGet&project references.
Would it be possible to enable this feature by default?

If this were to be an opt-out feature, what would be the opt-out mechanism?

I believe parallel parsing is on by default and the only way to turn it off for end-users is via the OtherFlags MSBuild property.

[vzarytovskii]

It should probably work the same way optimize works, with +/- suffix, where + is default for lang version preview (which will become 8).

[safesparrow]

where + is default for lang version preview (which will become 8).

Why would compilation parallelisation defaults be based on project's lang version? Do we do that for any other compilation performance flags?

Or do you mean version of SDK?

[vzarytovskii]

where + is default for lang version preview (which will become 8).

Why would compilation parallelisation defaults be based on project's lang version? Do we do that for any other compilation performance flags?

Or do you mean version of SDK?

Language version is the only one we can tie to, realistically.
My reasoning for having it on starting from 8 is to tie it to the new release, so we won't have any surprises and corner cases in sdk 7.x, since it will be much harder to backport fixes there going forward.