[NFCI][sanitizer_common] Realign #ifdefs in sanitizer_internal_defs.h (#186861)

Currently it is very hard to tell these nested ifdefs apart. This patch
fixes that, while trying to be as light-touch as possible.

GitOrigin-RevId: 6a994417d85116c4ae4f21f15c35f944944443fa
1 file changed