Extend clang lexer to support contract-keywords and gate behind -fverify-contracts

[SwayamInSync]

Summary

Extends Clang with first-class contract syntax keywords for deductive verification, gated behind the -fverify-contracts flag.

Changes

• Add new keywords to TokenKinds.def: pre, post, invariant, decreases, ghost, spec_fn, proof_fn
• Gate keywords with KEYCONTRACT flag, only active when -fverify-contracts is passed
• Add -fverify-contracts / -fno-verify-contracts driver and CC1 flags
• Add VerifyContracts to LangOptions
• Update IdentifierTable to handle contract keyword registration
• Add driver test (verify_contracts.cpp) and lexer test (verify_contracts_keywords.cpp)

Design

Keywords are invisible to normal compilation — they remain plain identifiers unless -fverify-contracts is explicitly passed. This ensures zero impact on existing C++ code.

[Copilot]

Pull request overview

Extends Clang’s lexer/identifier handling to recognize a set of “contract” keywords only when -fverify-contracts is enabled, and wires the new flag through the driver/cc1 along with targeted tests.

Changes:

• Add KEYCONTRACT gating and register new contract keywords in TokenKinds.def.
• Introduce LangOptions::VerifyContracts and connect keyword enablement to it via IdentifierTable.
• Add -fverify-contracts driver forwarding plus new driver/lexer tests.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
clang/test/Lexer/verify_contracts_keywords.cpp	Verifies keyword-vs-identifier lexing behavior with and without -fverify-contracts.
clang/test/Driver/verify_contracts.cpp	Ensures -fverify-contracts is forwarded to cc1 and is absent by default.
clang/lib/Driver/ToolChains/Clang.cpp	Forwards -fverify-contracts to cc1 using opt-in flag handling.
clang/lib/Basic/IdentifierTable.cpp	Enables/disables KEYCONTRACT keywords based on LangOptions::VerifyContracts.
clang/include/clang/Options/Options.td	Defines -fverify-contracts and maps it to LangOptions::VerifyContracts.
clang/include/clang/Basic/TokenKinds.def	Adds KEYCONTRACT and the contract keyword list.
clang/include/clang/Basic/LangOptions.def	Introduces the VerifyContracts language option.
clang/include/clang/Basic/IdentifierTable.h	Adds the KEYCONTRACT token key bit and updates KEYMAX.

[SwayamInSync]

We could've also added the build instructions but since the scope is limited of this PR (just lexer support) so to avoid confusion those can be happen at later mature stages

[SwayamInSync]

Also in a follow-up, it might be better to rename ghost {...} block to something like proof block?