[Coroutines] Do not add alloca to the frame if the size is 0

This patch is to address https://bugs.llvm.org/show_bug.cgi?id=49916.
When the size of an alloca is 0, it will trigger an assertion in OptimizedStructLayout when being added to the frame.
Fix it by not adding it at all. We return index 0 (beginning of the frame) for all 0-sized allocas.

Differential Revision: https://reviews.llvm.org/D101841