[mach-o] don't assume all sections have symbols

We were trying to examine the first symbol in a section that we wanted to
atomize by symbols, even when there wasn't one. Instead, we should make the
initial anonymous symbol cover the entire section in that situation.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@211681 91177308-0d34-0410-b5e6-96231b3b80d8
[mach-o]: atomize zero-terminated literals correctly.

When looking through sections with zero-terminated string-literals (__cstring
or __ustring) we were constantly rechecking the first few bytes of the string
for '\0' rather than advancing along. This obviously failed unless all strings
within the section had the same length as that first one.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@211682 91177308-0d34-0410-b5e6-96231b3b80d8