[ADT] Support const-qualified unique_functions
Summary:
This technique should extend to rvalue-qualified etc, but I didn't add any.
I removed "volatile" from the future plans, which seems... speculative at best.
While here I moved the callbacks object out of the constructor into a
variable template, which I believe addresses the fixme there about unused
objects.
(I'm not a template guru, so it's always possible the old version was designed
for compile-time performance in a way I'm missing)
Reviewers: kadircet
Subscribers: dexonsmith, llvm-commits, chandlerc
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D82581