[llvm] [UTC] Escape multiple {{ or }} in input for check lines. (PR #71790)

[Florian Hahn via llvm-commits]

fhahn wrote:

> Can you please add a UTC test for this (or adjust an existing one to exercise this)?

Thanks, added test in 129a91ec01a9.

https://github.com/llvm/llvm-project/pull/71790