[DebugInfo] Do not generate label debug info if it has been processed.

In DwarfDebug::collectEntityInfo(), if the label entity is processed in
DbgLabels list, it means the label is not optimized out. There is no
need to generate debug info for it with null position.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@341513 91177308-0d34-0410-b5e6-96231b3b80d8
8 files changed