Review visibility of compilation options

[tmat]

There are multiple comments in CompilationOptions.cs such as:

// TODO: change visibility of the ReferencesSupersedeLowerVersions setter to internal & protected

and properties with odd names like:

 internal DateTime CurrentLocalTime_internal_protected_set { set { CurrentLocalTime = value; } }
  internal bool DebugPlusMode_internal_protected_set { set { DebugPlusMode = value; } }

DebugPlusMode should also be public - it would make sense as a third value of OptimizationLevel rather then a separate property.
This is needed so that a tool can be written that reconstructs a compilation from DLL/PDB using public APIs only.

[tmat]

@jaredpar @clairernovotny @ryzngard