llvm: expose token none constant

[cpunion]

Summary

• expose llvm::ConstantTokenNone::get(Context) as Context.ConstTokenNone()
• keep the binding to one narrow C++ shim returning the context-owned constant
• verify the value has token type and prints as token none
• verify it can be used by llvm.coro.suspend and the version-appropriate llvm.coro.end signature

This is the missing primitive needed for LLVM 19 switched-resume coroutine lowering in llgo.

Validation

• full go test . with the default LLVM 19 configuration
• go vet .
• targeted verifier runs with LLVM 14, 18, 19, 20, and local LLVM 22
• focused CI covers LLVM 14/18/19/21 on Linux

[gemini-code-assist]

Code Review

This pull request introduces support for the LLVM ConstantTokenNone constant in the Go bindings. It adds a new C binding function LLVMGoConstTokenNone in IRBindings.cpp and IRBindings.h, exposes it as the ConstTokenNone method on the Context struct in ir.go, and includes a comprehensive test suite in ir_test.go to verify its integration with coroutine intrinsics across different LLVM versions. No review comments were provided, so there is no additional feedback to address.

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