As we've now finished parsing the constructs, we're moving onto
implementing 'clause' parsing. While some are complicated and require
their own patch, the handful added here are simple to parse (that is,
they are a single identifier).
This patch adds the infrastructure to parse these and a clause-list in
its entirety. This adds some complication to how we are diagnosing
parsing errors elsewhere, so a few changes were made to better recover
from errors.
The 'cache' construct takes a list of 'vars', which are array-section
style definitions. This patch implements the parsing, leaving the lower
bound and length of the bound as expressions, so that we can validate
they are the correct 'thing' in sema.
