libDebugInfo: Avoid independently parsing the same .dwo file for two separate CUs residing there

NFC, just an optimization. Will be building on this for DWP support
shortly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303591 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed