layers: Tweak PV generator to track newly introduced enums separately

layers: Tweak PV generator to track newly introduced enums separately

Within each feature block, we need to generate a helper value for 'all
valid bits'. This worked fine; however, across features blocks an enum
may be used -- for example, core eums used in extension functions.

Previously, we'd clear out all the enums we know about at the feature
boundary, which left various fields unvalidated.

Signed-off-by: Chris Forbes <chrisforbes@google.com>