Extract logic for escaping a string field in the new debug info
hierarchy from `GenericDebugNode`. A follow-up commit will use it far
more widely (hence the dead code for `ShouldSkipEmpty`).
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230873 91177308-0d34-0410-b5e6-96231b3b80d8