pattern-gen: introduce behavior AST scaffolding for frontend/backend split

[zhouguangyuan0718]

Motivation

• Separate behavior parsing front-end from LLVM IR emission to enable a clean frontend/backend split for CoreDSL Behavior handling.
• Represent statements and expressions explicitly (e.g. declarations, if/while/for, assignments, index operations) so future passes can traverse a structured AST instead of relying solely on immediate IR lowering.
• Keep existing LLVM IR generation intact during this refactor to preserve current behavior while introducing the new scaffold.

Description

• Add BehaviorExpression and BehaviorStatement structs to model expression kinds (Generic, Assignment, Index) and statement kinds (Scope, If, While, For, Declaration, Expr).
• Introduce classify_expression(...) which runs the existing ParseExpression pipeline and then heuristically classifies the parsed source slice (looking for [ or assignment = vs ==) into expression kinds.
• Thread an optional AST pointer through ParseDeclaration, ParseStatement, and ParseScope (added optional BehaviorStatement *ast parameter) so parsing can populate a lightweight AST while continuing to emit LLVM IR via the current code paths.
• In ParseBehaviour create a top-level BehaviorStatement behaviorAST and invoke ParseStatement(..., &behaviorAST), thereby constructing the new frontend representation without changing the current lowering that follows.

Testing

• Applied the patch successfully to llvm/tools/pattern-gen/lib/Parser.cpp and inspected the resulting diff with git diff, confirming the structural changes were introduced as intended.
• Committed the change with git -C /workspace/CoreDSL2LLVM commit ... and the commit succeeded.
• No full LLVM build or compile-time tests were executed in this environment because no build directory / configured LLVM build was available, so runtime/compile validation was not performed.

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Codex Task