Completion performance for multitargeted projects is slow

[cartermp]

With SDK-style projects, if you multi-target, the experience is subpar due to very slow completion. With #4889 this is alleviated for a time, but after a while it will still invalidate caches and rebuild chunks of the world. The language service itself needs to properly understand the additional context that multitargeting requires.

Although this is not a regression, it is still quite a bad experience for people who multitarget. We recommend sticking to a single target if you don't require multitargeting.

[TIHan]

This should help it even more: #4918

We will still type check multiple times, once per TFM; that is what Roslyn does. But, entire swaths of cache will not be invalidated anymore.

I tested this myself and now multi-targeting is much much better. Especially in Area51's case.

[TIHan]

This is now resolved.