[clang] [clang][CodeGen] Allow memcpy replace with trivial auto var init (PR #84230)

[Nikita Popov via cfe-commits]

nikic wrote:

This change also "fixes" https://github.com/llvm/llvm-project/issues/78034 in the sense that we return to the clang 17 status quo of the emitted IR being wrong but mostly working out in practice.

https://github.com/llvm/llvm-project/pull/84230