[DebugInfo] Replace unreachable with None

Invalid user input should not trigger assertions and unreachables. We
already return an Option so we should just return None here.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=5532

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@327274 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed