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 diff revision, where Diffs 1 and 2 have "alphabet.txt" with A-Z on one line each. The file is unchanged across diffs; some other file is changed.
• Added a comment to lines P-Z of Diff 1.
• Before: comment is adjusted out of range on Diff 2 and not shown in the UI.
• After: comment is still adjusted out of range internally, but now corrected into the display range and shown.

Differential Revision: https://secure.phabricator.com/D21435