LWG-3117 Missing packaged_task deduction guides

[futuarmo]

LWG-3117 Missing packaged_task deduction guides

I've got deduction guides from function, because there are similar requirements for packaged_task deduction guides. All tests are passed

Fixes #1465

[StephanTLavavej]

Thanks for the CTAD upgrade! I pushed a couple of changes:

1. Rename the macro being used to stamp out the packaged_task deduction guides for different calling conventions, so it's not the same as the macro being used for function. This makes it easier to search the codebase, even though there was no actual conflict (because we immediately #undef the macros).

2. Add test coverage. Since we already had good test coverage for function, I simply templatized it. The only extra work necessary (aside from guarding this against /clr and /clr:pure) was that function is copyable while packaged_task is movable-only. I used if constexpr to test copy construction when available for function, and move construction always.

[StephanTLavavej]

Thanks for making packaged_task easier to use! 📦 🐈 😸