Auto merge of #59500 - crlf0710:boxed-closure-impls, r=cramertj
Unsized rvalues: implement boxed closure impls. (2nd try) This is a rebase of S-blocked-closed PR #55431 to current master. LLVM has moved forward since then, so maybe we can check whether the new LLVM 8.0 version unblocked this work.
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