Make Portable PDB the default on Windows

VS 2015 Update 2 CTP has been released which has support for debugging portable PDBs.  Now is a good time to make the transition so a) we can root out any remaining issues with portable PDBs and b) have larger dogfooding of the VS debugging experience here.

closes #704

src/dotnet/commands/dotnet-compile-csc/Program.cs

@@ -128,13 +128,10 @@ namespace Microsoft.DotNet.Tools.Compiler.Csc
             var args = new List<string>()
             {
                 "-nostdlib",
-                "-nologo"
+                "-nologo",
+                "-debug:portable"
             };
 
-            args.Add(RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
-                ? "-debug:full"
-                : "-debug:portable");
-
             return args;
         }