Unbound wildcards cause assertion error/oom

[cswinter]

E.g. val a: _ = ??? causes compiler hang/OOM and val a: List[_ <: _] = List(0) crashes with a failed assertion in typer. Some more details on why this happens can be found in this comment. This can only be replicated with dotc; REPL aborts compilation as soon as the frontend emits an "unbound wildcard type" error.

Further examples can be found in tests/pending/neg/unboundWildcard.scala, which should be moved to tests/neg once this issues has been resolved.

[odersky]

I think this has an easy fix: When the parser reports an unbound wildcard error for a type, it should return Any as the tree instead of the TypeBounds tree that it saw originally. No need to worry about follow-on errors: the reporter will suppress subsequent errors on the same position anyway.