Auto merge of #117947 - Dirbaio:drop-llvm-15, r=cuviper

Update the minimum external LLVM to 16.

With this change, we'll have stable support for LLVM 16 and 17.
For reference, the previous increase to LLVM 15 was #114148

[Relevant zulip discussion](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/riscv.20forced-atomics)