Fix oversharing of build results in ResultsCache

[ladipro]

Fixes #9458

Context

#9565 had to be reverted because the assumption that results with BuildRequestDataFlags.ProvideSubsetOfStateAfterBuild don't have to be cached broke design-time builds in Visual Studio.

Changes Made

Re-did the change, now with full handling of BuildRequestDataFlags.ProvideSubsetOfStateAfterBuild.

Testing

New and existing unit tests, experimental insertion, manual testing with the problematic design-time build scenario.

Notes

Compared to #9565, we now:

• Copy the flags from the original BuildResult in the BuildResult copy constructor.
• Have ProjectInstance remember the project state filter it was created from.
• Implement IsSubsetOf operator on RequestedProjectState.
• Use the IsSubsetOf to determine if a request with ProvideSubsetOfStateAfterBuild can be satisfied by the cache.
• Don't consider SkipNonexistentTargets to be a flag affecting build results.

[ladipro]

I have removed SkipNonexistentTargets from the flags that have to match. This is a cache of target results so if the target is there, it has to exist. Re-tested everything and will merge tomorrow if there are no objections.