LWG2976: Dangling uses_allocator specialization for packaged_task

[ldionne]

Link: https://wg21.link/LWG2976

BEGIN-RST-NOTES
Originally implemented in LLVM 6 but reverted later. Old documentation incorrectly said it was implemented.
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[ldionne]

This issue has already been implemented and was created for tracking purposes only.

[llvmbot]

@llvm/issue-subscribers-c-20

Author: Louis Dionne (ldionne)

Details

**Link:** https://wg21.link/LWG2976