Fix dangling IPOAmendableCB function_ref. (llvm#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.

Author: hokein

llvm/include/llvm/Transforms/IPO/Attributor.h

@@ -1478,7 +1478,7 @@ struct AttributorConfig {
   /// The name of the pass running the attributor, used to emit remarks.
   const char *PassName = nullptr;
 
-  using IPOAmendableCBTy = function_ref<bool(const Function &F)>;
+  using IPOAmendableCBTy = std::function<bool(const Function &F)>;
   IPOAmendableCBTy IPOAmendableCB;
 };