[llvm][NFC] Cleanup uses of std::function in Inlining-related APIs

Summary:
Replacing uses of std::function pointers or refs, or Optional, to
function_ref, since the usage pattern allows that. If the function is
optional, using a default parameter value (nullptr). This led to a few
parameter reshufles, to push all optionals to the end of the parameter
list.

Reviewers: davidxl, dblaikie

Subscribers: arsenm, jvesely, nhaehnle, eraman, hiraditya, haicheng, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79917