Express Registers_*::lastDwarfReg using _LIBUNWIND_HIGHEST_DWARF_REGISTER

This avoids having to keep the same information duplicated in multiple
places.

Adjust _LIBUNWIND_HIGHEST_DWARF_REGISTER to actually have the value
of the highest used register and only use the value
_LIBUNWIND_HIGHEST_DWARF_REGISTER + 1 (kMaxRegisterNumber + 1) for
allocating the savedRegisters array.

Differential Revision: https://reviews.llvm.org/D39281

llvm-svn: 316745