[include-cleaner] clang-include-cleaner can print/apply edits

This adds command-line flags to the tool:
+ -print: prints changed source code
+ -print=changes: prints headers added/removed
+ -edit: rewrites code in place
+ -insert=0/-remove=0: disables additions/deletions for the above

These are supported by a couple of new functions dumped into Analysis:
analyze() sits on top of walkUsed and makes used/unused decisions for
Includes. fixIncludes() applies those results to source code.

Differential Revision: https://reviews.llvm.org/D139013