Remove DwarfDebug::FirstCU as it has no use

It was only being used as a flag to identify the lack of debug info from
within endModule - use the section labels for that instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220575 91177308-0d34-0410-b5e6-96231b3b80d8