[include-cleaner] HTMLReport shows headers that would be inserted

Demo: https://htmlpreview.github.io/?https://gist.githubusercontent.com/sam-mccall/9730f933a2cf2e003365520b6636f731/raw/7911d8251ceab7c244e0510285105027cd0a9403/PathMapping.cpp.html

Header insertion doesn't actually work that well (not this patch's fault):
- we don't have ranking of locations/headers yet, so inserted header is pretty
  random
- on my system, we get a lot of absolute "/usr/bin/../include/..." paths.
  This is a HeaderSearch bug introduced in D60873 that I'll send a fix for

Differential Revision: https://reviews.llvm.org/D138676