SI-8202 bug compatibility with SI-8211 for quasiquotes
Move placeholder construction logic into Placeholders

Previously construction logic used to be in Parsers and deconstruction in
Placeholders making it easy to forget one if you change the other.
Move null check case higher to avoid NPE

Also use sym.isErrorneous instead of manual name check.
Improve support for patterns in vals

This commits adds construction-only support for splicing patterns into
vals (a.k.a. PatDef). Due to non-locality of the desugaring it would
have been quite expensive to support deconstruction as the only way to
do it with current trees is to perform implodePatDefs transformation on
every single tree.
Merge pull request #3455 from densh/topic/patdef

Fix SI-8202 and improve support for splicing patterns into vals