e.g `val x2 = Foo[$1] with Bar = boundValue`

is rewritten to `val x2 = (Foo[$1] @uncheckedBounds) with Bar = boundValue`

This is to have refchecks turn a blind eye to the type argument that
doesn't conform the to type parameter bounds.

For regular compilation, without the async transform between patmat
and refchecks, bound conformance is disabled with:

  https://github.com/scala/scala/blob/v2.11.7/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala#L1743

Using the `uncheckedBounds` annotation is a newer, more inclusive way
of acheiving the same thing:

  https://github.com/scala/scala/blob/v2.11.7/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala#L1677