[xbuild] implement ToolLocationHelper.GetPathToStandardLibraries().
[xbuild] fix NET_2_0 build.
[ms.build] Add msbuild.rsp.
[ms.build] follow xbuild to use version 12.0.
[ms.build] use appropriate ProjectCollection so that it can build the projects with appropriate settings.

Now a solution turns into a project with "BuildingSolutionFile" property
which is assigned by SolutionParser and subsequently gets _ExplicitReference
that is to store path to mscorlib.dll and thence projects build.