Control persistence of PC doc comments.

Comments obtained by parsing are cleared at each typer run if `CommentPreservingTypers` isn't mixed in.

- fixes memory leak
- makes Scaladoc slower

It's possible (and easy : add `icUnit.scalaProject.presentationCompiler(_.clearDocComments())` right before [that line](https://github.com/scala-ide/scala-ide/blob/master/org.scala-ide.sdt.core/src/org/scalaide/ui/internal/reconciliation/ScalaReconcilingStrategy.scala#L48)) to reset those comments only at reconciliation, but the memory-leak test doesn't trigger reconciliation (it types all the time).

======

Tested with :
```
huitseeker➜Scala/scala-ide/org.scala-ide.sdt.build(hotfix/scaladoc-memoryleak✗)» mvn --projects \
../org.scala-ide.sdt.aspects,../org.scala-ide.sdt.core,../org.scala-ide.sdt.core.tests \
-Dscala211.version=2.11.3-SNAPSHOT -P scala-2.11.x,memory-test,eclipse-luna  clean integration-test
```

======
Edit : i've added a more involved fix as a second commit, which triggers docComment clearing at each reconciliation and make the Memleak test trigger reconciliation. LMK what you think about including it.