Create symbols marking the start of a section earlier.

Create symbols marking the start of a section earlier.

This lets us pass the symbol to the constructor and avoid the mutable field.

This also opens the way for outputting the symbol only when needed, instead
of outputting them at the start of the file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231859 91177308-0d34-0410-b5e6-96231b3b80d8