Stop using hardcoded numbers for -Z options

Instead use a macro and generate them!
Remove obsoleted -Z options

* borrowck_note_pure - unused
* borrowck_note_loan - unused
* no_debug_borrows - unused
* lint_llvm - equivalent to -Z no-prepopulate-passes + --llvm-passes lint
Purge borrowck from libstd

This hasn't been in use since `@mut` was removed
Capitalize debugging opts and make them u64