coroutine allocation optimization

[kelbon]

I was writing recursive generator and wanted to make a static 'empty generator'.
Because i cannot create promise by myself i create a lambda and static 'generator' which is always empty.

Unfortunately clang is not able to optimize generator allocation in such a trivial case, while 'static' means that compiler must be 100% sure its memory never dies

Example with task(co_await adds 'new' call, because compiler thinks memory for frame required):
https://godbolt.org/z/G9Mqa4zbj

So i propose to not allocate frame (through new) for static coroutines

[llvmbot]

@llvm/issue-subscribers-coroutines

[ChuanqiXu9]

Let's track the issue in #64586.