Add Microsoft mangling for _Float16

Enables _Float16 on Windows by creating a mangling
mechanism in MicrosoftMangle.  It accomplishes this by 
mangling as a structure type of __clang::_Float16, similar
to how Complex works.

Patch By: mibintc
Differential Revision: https://reviews.llvm.org/D45738

llvm-svn: 330225