[PATCH] [-cxx-abi microsoft] Mangle user defined entry points properly

[Reid Kleckner]

  There's one outstanding issue here, which is that now we generate invalid obj files if you overload one of these guys.  =/

  IMO we should have an equivalent of CheckMain which marks them all implicitly extern "C", and then let other machinery take care of the rest.

http://llvm-reviews.chandlerc.com/D1670

COMMIT
  http://llvm-reviews.chandlerc.com/rL190675