This adds support for a new ICompilerOptionsHostObject which is how we
can pass options through the project system without having to manually
add separate properties for everything as we've traditionally done in
the past. To ensure compatibility with "new" compiler tasks against
"old" project systems, and "new" project systems against "old" tasks, we
continue to use the historical ICscHostObject* and IVbcHostObject*
properties for all existing values, and only use this property to
shuttle through new values. For the rest of Visual Studio 2015, we can
just use this for any new "simple" switches that we must pass along.  We
still would need to do additional work in the project system for any
inputs that are lists of files because of the fast-up-to-date check.

Longer term, there is still the unanswered question of how we want to
future versions of Visual Studio to get command line options to
completely get rid of all language service knowledge of this. There are
two possible approaches:

1. We just use the support added in #4604 and just delete this entirely.

2. We continue to use ICompilerOptionsHostObject. My proposal for that
   would be we continue to use the model of "we pass all options not
   supported by a strongly typed host object interface to
   ICompilerOptionsHostObject", and the project
   system would just remove all support for everything else and
   fail those QueryInterface calls.

This is therefore written with the idea that we could switch to option 2
in a coordinated way, if we wanted to do so.

Fixes #7812, assuming you have an updated csproj.dll and msvbprj.dll
which understands the new ICompilerOptionsHostObject.