abitidy: extract code to get a comment node into a separate function am: 9ee33c13f9 am: 18560ea489 am: 94e310bc47

Original change: https://android-review.googlesource.com/c/platform/external/libabigail/+/2175523

Change-Id: I4f98b11462c631bb7d9bd07dd487ba713fe71ed4
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>