Consider setting TreatWarningsAsError for all projects

[Eilon]

After a few of the currently-open PRs are merged (#174, #178), all compiler and code analyzer warnings should be fixed. It would be good to set TreatWarningsAsErrors to true for all projects to ensure that contributors are forced to fix warnings before submitting a PR.

I recommend this setting be placed in the new Directory.Builds.props file that I included in this PR: https://github.com/actions/runner/pull/173/files#diff-6483fde6c45c90fdfe98ca72eadfa651

That way the setting will automatically be shared by all projects in that folder and sub-folders. Individual projects can override that setting if they want to (but they shouldn't!).