llvm-cov: Don't assume FileChecksum was generated.

For cases where emitProfileArcs() was called but emitProfileNotes() was
not, set the CfgChecksum to 0.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195311 91177308-0d34-0410-b5e6-96231b3b80d8