Add exception message to CSharpCompilation.GetSpecialType

Related to https://devdiv.visualstudio.com/DevDiv/_workitems/edit/459418.

Our non-fatal error reports indicate this method is often called with
an out-of-range value for the `specialType` parameter. This commit adds
a custom message to the resulting `ArgumentOutOfRangeException` so we
can see what we're being asked about.

src/Compilers/CSharp/Portable/Compilation/CSharpCompilation.cs

@@ -1253,7 +1253,7 @@ internal new NamedTypeSymbol GetSpecialType(SpecialType specialType)
         {
             if (specialType <= SpecialType.None || specialType > SpecialType.Count)
             {
-                throw new ArgumentOutOfRangeException(nameof(specialType));
+                throw new ArgumentOutOfRangeException(nameof(specialType), $"Unexpected SpecialType: '{(int)specialType}'.");
             }
 
             var result = Assembly.GetSpecialType(specialType);