[libc] fix readability-identifier-naming in memory_utils/utils.h (#83919)

Fixes:

    libc/src/string/memory_utils/utils.h:345:13: warning: invalid case style
    for member 'offset_' [readability-identifier-naming]

Having a trailing underscore for members is a google3 style, not LLVM style.
Removing the underscore is insufficient, as we would then have 2 members with
the same identifier which is not allowed (it is a compile time error). Remove
the getter, and just access the renamed member that's now made public.