Refactor how passes get a symbol at the end of a section.

Refactor how passes get a symbol at the end of a section.

There is now a canonical symbol at the end of a section that different
passes can request.

This also allows us to assert that we don't switch back to a section whose
end symbol has already been printed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233026 91177308-0d34-0410-b5e6-96231b3b80d8