[ModuleInliner] Add MLPriority

This patch adds MLPriority as the first step toward the ML-based
function inlining with the module inliner.

For now, MLPriority is completely identical to CostPriority.

Once this patch lands, I'm planning to:

- integrate NoInferenceModelRunner,

- memoize the priority computation so that the priority remains the
  same for given values of metrics even with the noise injected during
  training, and

- port/take more features into account.

Differential Revision: https://reviews.llvm.org/D139140