lld/include/lld/Core/TODO.txt

- fix minor grammar stuff (I'm not a native speaker either, but it's hopefully a net improvement)
- mention that lld/coff is used in production
- update AArch64, ARM to production quality
- remove lld/include/lld/Core/TODO.txt which looks outdated

Differential Revision: https://reviews.llvm.org/D56600

llvm-svn: 351030