This reverts commit b03e8478.

The flag name is "IOperation". Register opeartion actions and calling
SemanticModel.GetOperation while the feature is disabled will throw an
InvalidOperationException. Also log a non-fatal-watson when
SemanticModel.GetOperation crashes.

This reverts commit 7bb0b4ea.

This reverts commit d43d4780.

This reverts commit 57a34148.

This reverts commit 8108db57.

This reverts commit 760e1204.

This reverts commit e2fc5135.

This reverts commit eb1ede48.

This reverts commit e3c1ea08.

This reverts commit 8f26ffb5.

PR #8722 only had allow unsafe set in debug build, which is failing in
release. This sets allow unsafe for all configurations

This still only runs the tests on desktop (for now), but it makes
it possible to run them on coreclr.

Synchronize all corefx references to -23504 and bring in the
xunit coreclr test runner.

Since #f9b4d71a, tests inherit app.config if they don't specify one. Removed the ones that match the default behavior.

by removing `<DebugType>` lines from projects that import `VSL.Settings.targets`