Enable to parse enum constructors in annotations

[coord-e]

supports only simple cases in ad-hoc way.

I plan to re-implement annotations by lifting annotation expressions into Rust functions using macros. This will allow rustc to parse them into HIR, effectively delegating path resolution to the compiler.

[Copilot]

Pull request overview

This PR adds support for parsing enum constructors in annotations, enabling the use of datatype constructors like X::A(1) in Thrust's #[thrust::requires(...)] and similar annotations. The implementation is explicitly ad-hoc and limited in scope, with plans for a more comprehensive re-implementation using macros.

Key Changes:

• Added AnnotPath and AnnotPathSegment structs to represent paths in annotations
• Implemented path parsing to handle multi-segment paths with optional generic arguments
• Extended the annotation parser to recognize and process enum/datatype constructors

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
tests/ui/pass/annot_enum_simple.rs	Adds a passing test case demonstrating enum constructor usage in requires/ensures annotations
tests/ui/fail/annot_enum_simple.rs	Adds a failing test case to verify that incorrect enum constructor constraints are properly detected
src/annot.rs	Implements core parsing logic for paths and datatype constructors, including new structs AnnotPath and AnnotPathSegment, and new parsing methods

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