Add Python script to automatically update checks in lit tests

[azwolski]

Description

A script similar to llvm/utils/update_llc_checks.py (for IR lit tests). The script was mostly generated by an AI. It currently works only for basic cases, such as in this example. https://github.com/ispc/ispc/pull/3637

This is an early draft. I will clean up or rewrite it once I have specified all requirements and clearly defined the expected behavior.

Related Issue

• [ ] Linked to relevant issue(s)

Checklist

• [ ] Code has been formatted with clang-format (e.g., clang-format -i src/ispc.cpp)
• [ ] Git history has been squashed to meaningful commits (one commit per logical change)
• [ ] Compiler changes are covered by lit tests
• [ ] Language/stdlib changes include new functional tests for runtime behavior
• [ ] Documentation updated if needed

[aneshlya]

Since ispc supports multiple LLVM versions I'm not sure if this script will be widely used. When new LLVM version is supported, some of the lit tests should be updated but support of older LLVM versions should be preserved in them. I think it can be better achieved with Claude subagent than fixed script.