xhp field refactoring

Summary:
Allow spatch to read pfff.log, and remove the undefined xhp field
Workflow: run codegraph on www, with flag -no_fake_edge
while (still some bugs)
  use spatch to read in pfff.log, and fix one file at a time
  use fixer to generate diffs.
end

Test Plan:
cd tests/php/codegraph
make
cd ~/pfff
./spatch -remove_undefined_xhp_field tests/php/codegraph/pfff.log

Reviewers: pad

CC: platform-diffs@lists

Differential Revision: https://phabricator.fb.com/D869724
Merge branch 'xhp'

Conflicts:
	lang_php/analyze/foundation/graph_code_php.ml