Revert "Revert "Revert "Revert "MSBuild support fixes""""

This reverts commit c8b3f434f7795e6b0f4d4d290343381de191cadd.
Revert "Revert "Revert "Revert "[MSBuild] Implement RefreshWithContent in the v4 builder""""

This reverts commit c2567b318ae0655a019fa2638303836441fe0f52.
Revert "Revert "[MSBuild] Fix regression when building msbuild projects""

This reverts commit bc12ac115d37fbf151a7b087f7024608f4a238e2.
Fix build issue when using xbuild

Unload all unsaved projects before building, so that we use the saved
projects instead of the projects loaded in memory. This is a workaround to
a bug in xbuild.