PR24595: Ignore calling convention modifiers for structors in MS ABI.

MS compiler ignores calling convention modifiers for structors. This patch makes
clang do the same (for MS ABI). This fixes PR24595 and makes vswriter.h header
(from Windows SDK 8.1) compilable.

Differential Revision: http://reviews.llvm.org/D12402

llvm-svn: 247619