Updated builder-corrected-tag fixup to add placeholders always.

* echo "|foo" would parse to an empty string because this would
  parse to a tr-token which the builder strips but leaves behind
  a mw:StartTag meta-marker. findBuilderCorrectedTags function
  wasn't properly processing these markers and possibly other
  markers as well.

* Fixed addPlaceholderMeta helper function to be smarter and fix
  this up.  With this patch, this wikitext snippet now parses and
  RTs correctly.  There are notes in the file about possible
  improvements which makes this text editable (currently VE will
  likely ignore content wrapped in mw:Placeholder metas).

* 2 more wt2wt tests now green.

* Improves RT-ing of en:Drum tabs.

Change-Id: I259bd235b8b11bff706a30a13f29723c21cef892
Merge "Updated builder-corrected-tag fixup to add placeholders always."