Remove support for DWARF64.
LLVM doesn't produce DWARF64, and neither does GCC. LLDB's support
for DWARF64 is only partial, and if enabled appears to also not work.
Finally, it's untested. Removing this makes merging LLVM and
LLDB's DWARF parsing implementations simpler.
Differential Revision: https://reviews.llvm.org/D59235