Rollup merge of #100460 - cuviper:drop-llvm-12, r=nagisa

Update the minimum external LLVM to 13

With this change, we'll have stable support for LLVM 13 through 15 (pending release).
For reference, the previous increase to LLVM 12 was #90175.

r? `@nagisa`

diff --cc compiler/rustc_codegen_llvm/src/back/write.rs
Simple merge

diff --cc compiler/rustc_codegen_llvm/src/llvm/ffi.rs
Simple merge

diff --cc compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
Simple merge

diff --cc compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
Simple merge