The 'cache' construct is lowered as marking the acc.loop in ACC MLIR.
This results in any variable references that are not inside of the
acc.loop being invalid. This patch adds a warning to that effect, and
ensures that the variable references won't be added to the AST during
parsing so we don't try to lower them.
This results in loss of instantiation-diagnostics for these, however
that seems like an acceptable consequence to ignoring it.
This statement level construct takes no clauses and has no associated
statement, and simply labels a number of array elements as valid for
caching. The implementation here is pretty simple, but it is a touch of
a special case for parsing, so the parsing code reflects that.
