[llvm] [test-suite] Document the LLVM test-suite benchmark apps (PR #105843)

[Sam Parker via llvm-commits]

https://github.com/sparker-arm commented:

Seems like a good idea to me. Is it worth adding a column to state whether it's suitable for performance measurements? I guess my broader question would be whether that information is still encoded correctly in CMake too...

https://github.com/llvm/llvm-project/pull/105843