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

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hiraditya 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.

yeah please open a separate issue.

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