[include-cleaner] Add include-cleaner tool, with initial HTML report

The immediate goal is to start producing an HTML report to debug and explain
include-cleaner recommendations.
For now, this includes only the lowest-level piece: a list of the references
found in the source code.

How this fits into future ideas:
 - under refs we can also show the headers providing the symbol, which includes
   match those headers etc
 - we can also annotate the #include lines with which symbols they cover, and
   add whichever includes we're suggesting too
 - the include-cleaner tool will likely have modes where it emits diagnostics
   and/or applies edits, so the HTML report is behind a flag

Differential Revision: https://reviews.llvm.org/D135956