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

[via llvm-commits]

v01dXYZ wrote:

@hiraditya sorry i was tryharding I should have open an issue instead. Running lit don t rebuild anything. So the result for compile time are noisy and there is no way to estimate the variance and know if the number is statically significant.
That's quite a PITA for sharing reliable results to others.


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