[PGO] Add ability to mark cold functions as optsize/minsize/optnone (#69030)

The performance of cold functions shouldn't matter too much, so if we
care about binary sizes, add an option to mark cold functions as
optsize/minsize for binary size, or optnone for compile times [1]. Clang
patch will be in a future patch.

This is intended to replace `shouldOptimizeForSize(Function&, ...)`.
We've seen multiple cases where calls to this expensive function, if not
careful, can blow up compile times. I will clean up users of that
function in a followup patch.

Initial version: https://reviews.llvm.org/D149800

[1]
https://discourse.llvm.org/t/rfc-new-feature-proposal-de-optimizing-cold-functions-using-pgo-info/56388