[mach-o] Make anonymous atom out of section content before any symbol

In sections that are broken into atoms at symbols, if the first symbol in the
section is not at the start of the section, then make an anonymous atom for
the section content that is before the first symbol.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@210142 91177308-0d34-0410-b5e6-96231b3b80d8