[-cxx-abi microsoft] Correctly identify Win32 entry points

Summary:
This fixes several issues with the original implementation:
- Win32 entry points cannot be in namespaces
- A Win32 entry point cannot be a function template, diagnose if we it.
- Win32 entry points cannot be overloaded.
- Win32 entry points implicitly return, similar to main.

Reviewers: rnk, rsmith, whunt, timurrrr

Reviewed By: rnk

CC: cfe-commits, nrieck

Differential Revision: http://llvm-reviews.chandlerc.com/D1683