[clang] [Win32][Coroutine] Fix inalloca arguments lifetime in coroutines (PR #212331)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

We can't do this purely as a codegen thing; we have to model it in the AST, for a few reasons:

- There's a third state for constructors beyond "exists" and "doesn't exist": they can exist, but not be instantiated.
- Correctly calling a copy constructor is a lot more complicated in some edge cases, like with default arguments.
- Static analysis need explicit AST nodes to model the operations.

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Have you compared what MSVC does here?

https://github.com/llvm/llvm-project/pull/212331