Changed erasure of ThisType

Was: erasure of underlying type.
Now: erased typeRef of underlying class.

The differ if there is a self type. In this case,
a this type did erase to teh erasure of the self type, which
is wrong.
Fixed narrowLiftedOwner in LambdaLift

A lot of cases were missing before and caused failures
in the newly added test in TreeChecker#typedThis.

Now we assure that all this references appear inside
the referenced class.
Merge pull request #275 from dotty-staging/fix/#266-lambdaLift

Fix/#266 lambda lift