Move Repl check to a symbol method.

Having a SectionBase method check Repl is inconsistent with how we
handle other section information.

For example, if a section is replaced, Sec->Live is false and it is
natural for Sec->getOutputSection() to be null.

It is the symbol that is moved to the replacement section.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@320599 91177308-0d34-0410-b5e6-96231b3b80d8