Workaround for race condition during source indexing

Initialize the `TupleClass` symbol to prevent the presentation compiler
assertion that checks thread confinement to fail on parse trees, as
demonstrated by Re #1001986. This is a workaround for scalac ticket SI-8030, as
we need the fix to be readily available on Scala 2.10 as well.

Calling `initialize` is sufficient according to @retronym, as highlighted in
this comment https://github.com/scala/scala/pull/3262/files#r8361292
Merge pull request #608 from dotta/issue/workaround-for-1001986-take2

Workaround for race condition during source indexing