llvm-project says that it does not currently support pull requests and asks one to use https://reviews.llvm.org/ Now, I would like to submit my patch for this file. How would one proceed?
You should start small with something like a typo to get a feel for the submission and code review process. There is the My First Typo Fix doc for this. Also, watch the How to Contribute to LLVM tutorial from the 2019 Developer Meeting. You will also need reviewers. The LLVM Discord channel, git blame and the LLVM code owners can help here. Lastly, or maybe even firstly, you could post an RFC on the LLVM developers email list.