[lld-macho] Fix symbol name returned from InputSection::getLocation

This commit fixes the issue that getLocation always printed the name of
the first symbol in the section.

For clarity, upper_bound is used instead of a linear search for finding
the closest symbol name. Note that this change does not affect
performance: this function is only called when printing errors and
`symbols` typically contains a single symbol because of
.subsections_via_symbols.

Differential Revision: https://reviews.llvm.org/D127670