More MSVC cl and clang-cl fixes

[lukester1975]

Hi

There are few cl/clang-cl issues I've tripped over after #1061 (with VS17.8pre3):

• clang-cl doesn't yet implement [[msvc::no_unique_address]] (https://reviews.llvm.org/D110485), so things are currently very noisy (warning : unknown attribute 'no_unique_address' ignored [-Wunknown-attributes]) especially with the levels of expansion of STDEXEC_ATTRIBUTE...
• test_iterate doesn't build; missing a <numeric> include for std::accumulate
• __allocator is a #define in Windows SDK's shared/specstrings.h, so triggers stdexec/execution.hpp(273,25): error : expected unqualified-id. Maybe it's worth #include <windows.h> in a test(s) to highlight that? I've just hackily renamed the concept to __allocator_c.

Anyway all trivial but if it's any use: https://github.com/lukester1975/stdexec/tree/win-fixes

HTH

Thanks!

[maikel]

Cool! I think it would help if you could open a PR with your branch. Is it possible for you?

Thank you for your notes