Auto merge of #109474 - nikic:llvm-16-again, r=cuviper

Upgrade to LLVM 16, again

Relative to the previous attempt in rust-lang/rust#107224:
 * Update to GCC 8.5 on dist-x86_64-linux, to avoid std::optional ABI-incompatibility between libstdc++ 7 and 8.
 * Cherry-pick llvm/llvm-project@96df79a.
 * Cherry-pick llvm/llvm-project@6fc670e.

r? `@cuviper`

Author: bors