Handle vendored descriptor.proto gracefully in lowering

[emcfarlane]

Builds on #699, which replaced panics with a single valid flag. That flag was too coarse: any missing symbol blocked lowering for every file in the session, so a proto2/proto3 file against a pre-editions vendored descriptor.proto would fail compilation even though it used nothing editions-era.

Changes:

• Widen builtin:"optional" to cover everything introduced in editions 2023+: FeatureSet, all Feature* / Features, OptionTargets, UnverifiedLazy, EditionDefaults, EditionSupport*, ExtnDecls*.
• Replace the valid flag with one error diagnostic per missing required symbol, attached to descriptor.proto. Removes File.Lowered().
• IsClosedEnum / IsPacked / IsUnicode / populateJSONNames use syntax for proto2/proto3 and only consult FeatureSet for editions.
• Augment the "cannot find message field" diagnostic when the name matches a tagged-optional builtin, pointing at descriptor.proto and explaining it's editions-era.

Four fixtures in experimental/ir/testdata/builtins/ cover broken, vendored-proto3 (existing), vendored-proto3 with options, and vendored-editions with unavailable feature.

[doriable]

This looks generally good -- I left a note to include the symbol tables for the test cases, but am fine to approve ahead of those being generated. Thank you!