Remove most special treatment of specialized signatures.

[DanielRosenwasser]

Fixes #5659, #6276, and resolves the main issues in #6143

With this change, the only thing that makes a specialized overload "special" is that we will still bubble those signatures to the top for overload resolution. This is done in reorderCandidates.

Specifically, here is the new behavior:

• A specialized overload no longer needs to be assignable to one of its overloads. (thanks to Implement new overload compatibility checking #6075).
• A specialized overload must now be compatible with an implementation signature if one exists, in the same manner that all other overloads must be compatible.
• Specialized overloads are now considered in relation checks, whereas before, they were entirely ignored.

Edit on 2016-01-29: Fixes #6732.

[DanielRosenwasser]

@ahejlsberg @RyanCavanaugh could you guys take a look at this PR?

[ahejlsberg]

#6732 convinces me we need to get rid of the special treatment. So I'm good with merging this (once you fix the failed integration checks).

[DanielRosenwasser]

@ahejlsberg are we going to port this into 1.8 as well?