[llvm] [Workflows] Improve GitHub Actions Security hardening (PR #117787)

[via llvm-commits]

github-advanced-security[bot] wrote:

This pull request sets up GitHub code scanning for this repository. Once the scans have completed and the checks have passed, the analysis results for this pull request branch will appear on [this overview](/llvm/llvm-project/security/code-scanning?query=pr%3A117787+is%3Aopen). Once you merge this pull request, the 'Security' tab will show more code scanning analysis results (for example, for the default branch). Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results. For more information about GitHub code scanning, check out [the documentation](https://docs.github.com/code-security/code-scanning/introduction-to-code-scanning/about-code-scanning). 

https://github.com/llvm/llvm-project/pull/117787