MS ABI: Implement copy-ctor closures, finish implementing throw

This adds support for copy-constructor closures.  These are generated
when the C++ runtime has to call a copy-constructor with a particular
calling convention or with default arguments substituted in to the call.

Because the runtime has no mechanism to call the function with a
different calling convention or know-how to evaluate the default
arguments at run-time, we create a thunk which will do all the
appropriate work and package it in a way the runtime can use.

Differential Revision: http://reviews.llvm.org/D8225

llvm-svn: 231952