Preserve custom section names when coalescing.

The mergeByContent attribute on DefinedAtoms triggers the symbol table to
coalesce atoms with the exact same content. The problem is that atoms can also
have a required custom section. The coalescing should never change the custom
section of an atom.

The fix is to only consider to atoms to have the same content if their
sectionChoice() and customSectionName() attributes match.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@218893 91177308-0d34-0410-b5e6-96231b3b80d8