Update host compiler to LLVM 16

This updates the host compiler for dist-x86_64-linux to LLVM 16,
pulling in the BOLT fix at
llvm/llvm-project@1de305d,
which is needed to update Rust to LLVM 16.

src/ci/docker/host-x86_64/dist-x86_64-linux/build-clang.sh

@@ -4,7 +4,7 @@ set -ex
 
 source shared.sh
 
-LLVM=llvmorg-15.0.0
+LLVM=llvmorg-16.0.0-rc4
 
 mkdir llvm-project
 cd llvm-project