[lld-macho][nfci] Don't include null terminator in StringRefs

So @keith observed
[here](https://reviews.llvm.org/D128108#inline-1263900) that the
StringRefs we were returning from `CStringInputSection::getStringRef()`
included the null terminator in their total length, but regular
StringRefs do not. Let's fix that so these StringRefs are less confusing
to use.

Reviewed By: #lld-macho, keith, Roger

Differential Revision: https://reviews.llvm.org/D133728