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Richard Smith
d37f1ae6b5
Based on #5948. A couple of tricky parts: * When generating the C++ side of the thunk, we are given a pointer to the location to emplace the return value. The only mechanism C++ provides to perform this emplacement is using placement `operator new`, which requires a library function in the `<new>` header. We handle this by declaring that library function ourselves, and rely on Clang not actually needing a definition for it (which the standard library owns). * On the Carbon side of the thunk, we want to form an initializing expression as the result of the call. We don't have a way of expressing in SemIR that an initializing expression performs its initialization by storing through a pointer, so this PR adds a new initializing instruction, `InPlaceInit`, to model an initialization that's performed opaquely in-place.