[clang] [clang] Generate appropriate assume in presence of libc's memcpy (PR #119704)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

On the clang side, we've intentionally ignored the nonnull marking on memcpy and friends for a long time; we made a decision a few years back the optimization wasn't worthwhile compared to the security risk.

The updated patch doesn't do anything useful; we can prove the pointers must point to valid memory from the llvm.memcpy call itself.

https://github.com/llvm/llvm-project/pull/119704