Recognize direct properties access in ProjectInstance

[JanKrivanek]

Fixes #10604

Context

MSBuildTreatWarningsAsErrors access was not recognized by PropertiesCheck - since it was bypassing Expander.

Changes Made

Added routing of properties access to ProjectInstance

Testing

Added unit test case specifically for MSBuildTreatWarningsAsErrors (fails without fix)

Notes

Not handling accesses via Project - as that would get much more involved and is specific for API calls and VS - which are both currently unsupported scenarios

[rainersigwald]

I think in my head I was envisioning this being a new wrapper around ProjectInstance.GetPropertyValue() that had tracking, used only in the parts of the engine that pay special attention to magic properties. Is that something you considered?

[JanKrivanek]

I think in my head I was envisioning this being a new wrapper around ProjectInstance.GetPropertyValue() that had tracking, used only in the parts of the engine that pay special attention to magic properties. Is that something you considered?

I thought about it, then dismissed it and now on the second thought I decided to follow it :-)

Originaly I wanted to have it more universal for cases where msbuild might be called through API (as prep for VS integration). However that will anyway need much more other work (especially as evaluation is separated) - so let's handle the case that is problematic now