(stable) When parsing HTML in remarkup, always start a new tag when encountering "<"

Summary:
Depends on D20630. See https://discourse.phabricator-community.org/t/bug-in-src-parser-html-phutildomnode-php/2882/. Currently, we parse <x>1<2</x> as an <x> tag containing a <2</x> tag.

A more expected interpretation of this is a single <x> tag containing the content 1<2.

When we encounter a <, always start a new tag, even if we're currently in a tag.

Test Plan: Added a test and made it pass.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20631