Merge pull request dotnet/coreclr#5001 from AlexGhiondea/ReadAppContextOverrides

Introduce a way to override AppContext switches from the project.json

Commit migrated from https://github.com/dotnet/coreclr/commit/456e30ed5c734f7cc803105f1fdbdfa9fac10c89

diff --cc src/coreclr/src/mscorlib/mscorlib.shared.sources.props
Simple merge