pdl: use explicit diff labels

This lets us have more meaningful diffs: before they would always say
“left” and “right” and I noticed that I had a hard time remembering
which is which when trying to patch the code to match the output.

Bug: 228307941
Test: atest pdl_inline_tests
Change-Id: I19d7cbc52dcce67df2964045c753fb773ab15566