Model Spicy and HILTI attributes separately

[bbannier]

Current HILTI attribute::Kind holds both Spicy as well as HILTI attributes (e.g., parse-at cannot appear in HILTI code). We should attempt to model them separately; this might not be trivial since in C++ enums cannot be inherited.

I realized we have another issue with the new Kind enum: Spicy-side values now reside inside HILTI headers, which isn't great (things like MaxSize and ParseAt is nothing that HILTI should know about). Not immediately sure how to fix that, it's really like we'd need two enum types.

Originally posted by @rsmmr in #1936 (comment)