Add line mappings to project TagHelper attribute values.

- We now create LineMappings for instances where a TagHelper's attribute value is not of a string type.
- This will enable the Razor editor to create projections from .cshtml => .cs for TagHelper attributes.
- Modified the TagHelperCodeGenerator and TagHelperBlockBuiler to accurately track the attribute values start locations so it could flow into code generation.
- Modified existing tests to account for the new line mappings.

#207

/cc @dougbu 
Purposefully not creating the LinePragmas here. Attributes cannot be debugged normally in Razor and after some talks with Damian we've decided to work on supporting that when we address debugability for ordinary Razor attribute code. 

The issue in making TagHelpers attributes debuggable ends up being the same problem as normal HTML attributes: The line that starts the generated code is variable length and therefore doesn't result in a good debugging experience (badly placed yellow highlight).