[llvm] Document that llvm-lit needs to be run to collect static metrics (PR #94836)

[via llvm-commits]

v01dXYZ wrote:

Yes, actually `compile_time` metrics has already produced by running make/ninja and `lit` is just aggregating the `.o.time` per directory. Cf litsupport/modules/compiletime

Just to ask because I didn t find it, how to have a repeated build to have at least a averaged `compile_time` ? When we use perf there is the `-r` option but the wrapper script timeit.sh don t allow to add any options. 

https://github.com/llvm/llvm-project/pull/94836