Add DesignTimeBuild=true to legacy build from ProjectSystem

[KevinRansom]

This replaces: PR #4395.

Fixes: #3222

The project system wasn't setting the DesignTimeBuild flag when running design time builds. This PR addresses that issue.

[KevinRansom]

@forki this does not impact dotnet sdk projects.

[saul]

Out of interest, does this improve solution load times? May be worth checking with that “Huge” benchmark solution Ive posted a few times.

[0x53A]

@KevinRansom I think this is related to #1559.

In that issue, $(SolutionDir) is unset, I would guess that it would also need to be set here?

[Pilchie]

@davkean can you take a look at this too?

[KevinRansom]

@0x53A I can take a look separately at the $(SolutionDir) issue. But it seems like that would not be something the project system is supposed to provide.

I imagine that this may speed up performance load for legacy projects. I will take a look once I get tests passing.

Kevin

[cartermp]

@KevinRansom I believe the stress test that @saul mentioned is located here: https://github.com/Microsoft/visualfsharp/tree/master/tests/projects/stress

So it should be pretty easy to measure against baselines under notes.txt

[KevinRansom]

@0x53A Looks like a 10-12 % reduction in time when loading huge dense solution.

from (51.40, 52.51, 51.97, 51.96) seconds to (45.51, 45.68, 45.10, 45.32) seconds.

Time to semantic colorization.

[KevinRansom]

@0x53A

This should be in tomorrows nightly build drop. Since I am about to pull it. It won't make it into 15.6, but should first appear in a 15.7 preview when they come on stream.

Thanks for sticking at this, it yielded a nice improvement.

Kevin

[dsyme]

Can someone explain what setting the DesignTimeBuild flag does please, roughly speaking? Thanks!