Fix to bringforward.

Always bring forward in initial phase. Fixes a stale symbol error discovered when compiling most
parts of dotc when turning erasure on by default.
Fix to baseClasses.

Need to be invalidated on new run along with superClassButs and baseTypes. Fixes a stale symbol error discovered when compiling dotc.core with erasure turned on.
Fixes to erasure

Makes erasure pass the test suite.

Erasure is not yet turned turned on by default, because TestNonCyclic fails
with a stale symbol error. The problem is that This types are coupled to Symbols
and therefore don't reload. This is a problem is This types refer to static symbols
that get recompiled. We either have to drop using This types for static references,
or redefine thme so that can be reloaded.