CodeView: Provide a .def file with the register ids

The list of register ids was previously written out in a couple of dirrent
places. This puts it in a .def file and also adds a few more registers (e.g.
the x87 regs) which should lead to more readable dumps, but I didn't include
the whole list since that seems unnecessary.

X86_MC::initLLVMToSEHAndCVRegMapping is pretty ugly, but at least it's not
relying on magic constants anymore. The TODO of using tablegen still stands.

Differential revision: https://reviews.llvm.org/D38480

llvm-svn: 314821