Add DesignTime code generation support for TagHelpers.

- We now generate a subset of the TagHelper runtime code during DesignTime. This enables users to see errors in the editor
- Added tests to validate design time code generation.
- Refactored runtime code generation tests (we now use a lot of their infrastructure for the design time pieces).

#208

/cc @dougbu 
It ensures no conflicts between tag helper types.  Just because there's 2 `InputTagHelper` types doesn't mean their `.Type`s are of the same clr type.