R1:dbdfac1e072e

Recover inline comments which are "adjusted" off the end of a diff

Summary:
See PHI1834. Currently, the inline adjustment engine can sometime "adjust" an inline off the end of a diff. If it does, we lay it out on an invalid display line here and never render it.

Instead, make sure that layout never puts a comment on an invalid line, so the UI is robust against questionable decisions by the adjustment engine: no adjustment should be able to accidentally discard an inline.

Test Plan:
- Created a two…