DebugInfo: Avoid recreating matching labels in disparate places.

Instead, reuse the same MCSymbol - this should make the code easier to
follow by avoiding hard to trace dependencies between different bits of
code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196392 91177308-0d34-0410-b5e6-96231b3b80d8