SI-8030 Restore thread safety to the parser

In addition to the invariant "parsing doesn’t enter new symbols",
which was respected and tested in 760df9843a910d6, we also must
ensure that "parsing doesn't call Symbol#info".

That happend indirectly if we call `companionModule`.

This commit just converts the name of `class TupleN` to a term
name, rather than getting the name of its companion.

No test is included. This is tested upstream in:

    https://jenkins.scala-ide.org:8496/jenkins/view/Memory%20Leak%20Tests/job/scalac-memory-leaks-test-2.11.0/

Review by @densh / @dragos