Fix dangling IPOAmendableCB function_ref. (#120698)
The `IPOAmendableCB`'s type is `llvm::function_ref`, it is error-prone
to write code (e.g.
https://github.com/llvm/llvm-project/blob/5656cbca52545e608f6fb8b7c9a778c7c9b4b468/llvm/lib/Transforms/IPO/OpenMPOpt.cpp#L5812)
that assign a temporary lambda to an `IPOAmendableCB` object, which is a
use-after-free issue.
This patch changes the `IPOAmendableCB` to `std::function`, to avoid the
dangling issue.
