This makes code that uses section relative expressions (debug info) simpler and
less brittle.
This is still a bit awkward as the symbol is created late and has to be
stored in a mutable field.
I will move the symbol creation earlier in the next patch.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231802 91177308-0d34-0410-b5e6-96231b3b80d8