Auto merge of #109326 - nikic:revert-llvm-16, r=the8472

Revert "Auto merge of #107224 - nikic:llvm-16, r=cuviper"

This reverts commit 4a04d08, reversing changes made to 2d0a7de.

The LLVM 16 upgrade is almost certainly responsible for the CI segfaults that were observed after it landed, looking something like this:
```
Building stage1 library artifacts (x86_64-unknown-linux-gnu)
error: failed to run `rustc` to learn about target-specific information

Caused by:
  process didn't exit successfully: `/checkout/obj/build/bootstrap/debug/rustc - --crate-name ___ --print=file-names -Csymbol-mangling-version=legacy -Zunstable-options -Zunstable-options '--check-cfg=values(bootstrap)' '--check-cfg=values(stdarch_intel_sde)' '--check-cfg=values(no_fp_fmt_parse)' '--check-cfg=values(no_global_oom_handling)' '--check-cfg=values(no_rc)' '--check-cfg=values(no_sync)' '--check-cfg=values(freebsd12)' '--check-cfg=values(backtrace_in_libstd)' '--check-cfg=values(target_env,"libnx")' '--check-cfg=values(target_arch,"asmjs","spirv","nvptx","xtensa")' -Zmacro-backtrace -Clink-args=-Wl,-z,origin '-Clink-args=-Wl,-rpath,$ORIGIN/../lib' -Csplit-debuginfo=off -Cprefer-dynamic -Cembed-bitcode=yes '-Zcrate-attr=doc(html_root_url="https://doc.rust-lang.org/nightly/")' --target x86_64-unknown-linux-gnu --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=split-debuginfo --print=crate-name --print=cfg` (exit status: 254)
  --- stderr
  /checkout/obj/build/x86_64-unknown-linux-gnu/stage1/lib/librustc_driver-fdca23ff32bf7124.so(+0x97f0b3)[0x7faee84e60b3]
  /lib/x86_64-linux-gnu/libc.so.6(+0x43090)[0x7faee77f2090]
  /checkout/obj/build/x86_64-unknown-linux-gnu/stage1/lib/libLLVM-16-rust-1.70.0-nightly.so(+0x7262e06)[0x7faee608fe06]
  /checkout/obj/build/x86_64-unknown-linux-gnu/stage1/lib/librustc_driver-fdca23ff32bf7124.so(+0xd10ad4)[0x7faee8877ad4]
  /checkout/obj/build/x86_64-unknown-linux-gnu/stage1/lib/librustc_driver-fdca23ff32bf7124.so(+0xbcb6cc)[0x7faee87326cc]
  /checkout/obj/build/x86_64-unknown-linux-gnu/stage1/lib/librustc_driver-fdca23ff32bf7124.so(+0xbca804)[0x7faee8731804]
  /checkout/obj/build/x86_64-unknown-linux-gnu/stage1/lib/librustc_driver-fdca23ff32bf7124.so(_RNvXs5_Cs7DTEPoQIU0M_18rustc_codegen_llvmNtB5_18LlvmCodegenBackendNtNtNtCsiydnvZ3aRh1_17rustc_codegen_ssa6traits7backend14CodegenBackend15target_features+0x1b)[0x7faee879183b]
  /checkout/obj/build/x86_64-unknown-linux-gnu/stage1/lib/librustc_driver-fdca23ff32bf7124.so(_RNvNtCsbMR7lNZiyEF_15rustc_interface4util17add_configuration+0x37)[0x7faee85d38f7]
  /checkout/obj/build/x86_64-unknown-linux-gnu/stage1/lib/librustc_driver-fdca23ff32bf7124.so(_RNvNtCsbMR7lNZiyEF_15rustc_interface4util14create_session+0x89d)[0x7faee85d439d]
  /checkout/obj/build/x86_64-unknown-linux-gnu/stage1/lib/librustc_driver-fdca23ff32bf7124.so(+0x989600)[0x7faee84f0600]
  /checkout/obj/build/x86_64-unknown-linux-gnu/stage1/lib/librustc_driver-fdca23ff32bf7124.so(+0x984380)[0x7faee84eb380]
  /checkout/obj/build/x86_64-unknown-linux-gnu/stage1/lib/librustc_driver-fdca23ff32bf7124.so(+0x985ec3)[0x7faee84ecec3]
  /checkout/obj/build/x86_64-unknown-linux-gnu/stage1/lib/libstd-2268eb07c4da6a07.so(rust_metadata_std_e292dd3bffb96032+0xca4ae)[0x7faee7a6f4ae]
  /lib/x86_64-linux-gnu/libpthread.so.0(+0x8609)[0x7faedec0d609]
  /lib/x86_64-linux-gnu/libc.so.6(clone+0x43)[0x7faee78ce133]
  [RUSTC-TIMING] ___ test:false 0.203
  rustc exited with signal: 11 (SIGSEGV) (core dumped)
```

r? `@ehuss`

Author: bors

.gitmodules

@@ -25,7 +25,7 @@
 [submodule "src/llvm-project"]
 	path = src/llvm-project
 	url = https://github.com/rust-lang/llvm-project.git
-	branch = rustc/16.0-2023-03-06
+	branch = rustc/15.0-2022-12-07
 [submodule "src/doc/embedded-book"]
 	path = src/doc/embedded-book
 	url = https://github.com/rust-embedded/book.git

src/llvm-project

@@ -38,10 +38,10 @@ pub fn short_integer_zip_map(x: [u32; 8], y: [u32; 8]) -> [u32; 8] {
 //
 // CHECK-LABEL: @long_integer_map
 #[no_mangle]
-pub fn long_integer_map(x: [u32; 512]) -> [u32; 512] {
+pub fn long_integer_map(x: [u32; 64]) -> [u32; 64] {
     // CHECK: start:
-    // CHECK-NEXT: alloca [512 x i32]
-    // CHECK-NEXT: alloca %"core::mem::manually_drop::ManuallyDrop<[u32; 512]>"
+    // CHECK-NEXT: alloca [64 x i32]
+    // CHECK-NEXT: alloca %"core::mem::manually_drop::ManuallyDrop<[u32; 64]>"
     // CHECK-NOT: alloca
     // CHECK: mul <{{[0-9]+}} x i32>
     // CHECK: add <{{[0-9]+}} x i32>