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.
438863a09e