MS ABI: Implement x86_64 RTTI

Summary:
The RTTI scheme for x86_64 is largely the same as the one for i386.

Differences are largely limited to avoiding load-time relocations by
replacing pointers to RTTI metadata with the difference of that data
relative to the load address of the module.

Interestingly, this precludes the possibility of successfully using RTTI
data from another DLL.  The ImageBase reference is always relative to
the current DLL.

Differential Revision: http://reviews.llvm.org/D4148