[lld-macho] Have path-related functions return std::string, not StringRef

[lld-macho] Have path-related functions return std::string, not StringRef

findLibrary() returned a StringRef while findFramework & other helper
functions returned std::strings. Standardize on std::string.

(I initially tried making the helper functions all return StringRefs,
but I realized we shouldn't return input StringRefs since their
lifetimes would not be obvious from the calling code.)