[NFC, Refactor] Rename the (scoped) enum DeclaratorContext's enumerators to remove duplication

Since these are scoped enumerators, they have to be prefixed by DeclaratorContext, so lets remove Context from the name, and return some characters to the multiverse.

Patch was reviewed here: https://reviews.llvm.org/D91011

Thank you to aaron, bruno, wyatt and barry for indulging me.