[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

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