Calling coroutines should be disallowed from [[nonallocating]] performance constraint setting

[cjappl]

C++20 coroutines allocate when beginning execution (source: Coroutines (C++20) - cppreference.com)

the coroutine state, which is internal, dynamically-allocated storage (unless the allocation is optimized out) …

…
When a coroutine begins execution, it performs the following:

allocates the coroutine state object using operator new.

The new performance constraints attributes ([[clang::nonallocating]] and stricter) should detect and warn about coroutines being called from a nonallocating context.

CC @dougsonos (I can't assign you because you aren't a member, but consider yourself "assigned")

[cjappl]

I put up a PR with a test to ensure that RTSan catches these allocations:

#111049

[llvmbot]

@llvm/issue-subscribers-coroutines

Author: Chris Apple (cjappl)

C++20 coroutines allocate when beginning execution (source: [Coroutines (C++20) - cppreference.com](https://en.cppreference.com/w/cpp/language/coroutines))

>the coroutine state, which is internal, dynamically-allocated storage (unless the allocation is optimized out) …

>…
>When a coroutine begins execution, it performs the following:

>allocates the coroutine state object using operator new.

The new performance constraints attributes ([[clang::nonallocating]] and stricter) should detect and warn about coroutines being called from a nonallocating context.

CC @dougsonos (I can't assign you because you aren't a member, but consider yourself "assigned")