[WinX86_64 ABI] Treat C99 _Complex as a struct

MSVC does not support C99 _Complex.
ICC, however, does support it on windows x86_64, and treats it, for purposes of parameter passing, as equivalent to a struct containing two fields (for the real and imaginary part). 

Differential Revision: http://reviews.llvm.org/D7825

llvm-svn: 230315