[pseudo] Perform unconstrained reduction prior to recovery.

Our GLR uses lookahead: only perform reductions that might be consumed by the
shift immediately following. However when shift fails and so reduce is followed
by recovery instead, this restriction is incorrect and leads to missing heads.

In turn this means certain recovery strategies can't be made to work. e.g.
```
ns := NAMESPACE { namespace-body } [recover=Skip]
ns-body := namespace_opt
```
When `namespace { namespace {` is parsed, we can recover the inner `ns` (using
the `Skip` strategy to ignore the missing `}`). However this `namespace` will
not be reduced to a `namespace-body` as EOF is not in the follow-set, and so we
are unable to recover the outer `ns`.

This patch fixes this by tracking which heads were produced by constrained
reduce, and discarding and rebuilding them before performing recovery.

This is a prerequisite for the `Skip` strategy mentioned above, though there are
some other limitations we need to address too.

Reviewed By: hokein

Differential Revision: https://reviews.llvm.org/D130523