Auto merge of #94381 - Kobzol:llvm-bolt, r=Mark-Simulacrum

Use BOLT in CI to optimize LLVM

This PR adds an optimization step in the Linux `dist` CI pipeline that uses [BOLT](https://github.com/llvm/llvm-project/tree/main/bolt) to optimize the `libLLVM.so` library built by boostrap.

Steps:
- [x] Use LLVM 15 as a bootstrap compiler and use it to build BOLT
- [x] Compile LLVM with support for relocations (`-DCMAKE_SHARED_LINKER_FLAGS="-Wl,-q"`)
- [x] Gather profile data using instrumented LLVM
- [x] Apply profile to LLVM that has already been PGOfied
- [x] Run with BOLT profiling on more benchmarks
- [x] Decide on the order of optimization (PGO -> BOLT?)
- [x] Decide how we should get `bolt` (currently we use the host `bolt`)
- [x] Clean up

The latest perf results can be found [here](https://github.com/rust-lang/rust/pull/94381#issuecomment-1258269440). The current CI build time with BOLT applied is around 1h 55 minutes.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/bootstrap/lib.rs b/src/bootstrap/lib.rs
index 9ada3f315c1..7e70e99bb8c 100644
--- a/src/bootstrap/lib.rs
+++ b/src/bootstrap/lib.rs
@@ -122,6 +122,7 @@ use crate::util::{
     check_run, exe, libdir, mtime, output, run, run_suppressed, try_run, try_run_suppressed, CiEnv,
 };
 
+mod bolt;
 mod builder;
 mod cache;
 mod cc_detect;