Fix memory errors with some --debug program builds

When we upgraded to LLVM 15, we accidentally changed our optimization
level for programs compiled with --debug. This uncovered a pre-existing
condition and caused issues like #4369.

This takes us back to LLVM 14 level but doesn't "fix" the underlying
issue which we are continuing to work on.

.release-notes/4369.md

@@ -0,0 +1,5 @@
+## Fix memory errors with some `--debug` program builds
+
+When we upgraded to LLVM 15, we accidentally changed the optimization level applied to `--debug` builds. That change in optimization levels surfaced a rather complicated bug that we've been looking into. The end result? Some programs would crash with memory errors like segfaults or bad access.
+
+We've updated to go back to the optimization setup we had with LLVM 14. We recommend updating your `ponyc` installation as soon as possible.

src/libponyc/codegen/genopt.cc

@@ -1013,9 +1013,13 @@ static void optimise(compile_t* c, bool pony_specific)
 
   // Create the top-level module pass manager using the default LLVM pipeline.
   // Choose the appropriate optimization level based on if we're in a release.
-  ModulePassManager MPM = PB.buildPerModuleDefaultPipeline(
-    c->opt->release ? OptimizationLevel::O3 : OptimizationLevel::O1
-  );
+  ModulePassManager MPM;
+
+  if (c->opt->release) {
+    MPM = PB.buildPerModuleDefaultPipeline(OptimizationLevel::O3);
+  } else {
+    MPM = PB.buildO0DefaultPipeline(OptimizationLevel::O0);
+  }
 
   // Run the passes.
   MPM.run(*unwrap(c->module), MAM);

src/libponyc/codegen/host.cc

@@ -31,9 +31,7 @@ LLVMTargetMachineRef codegen_machine(LLVMTargetRef target, pass_opt_t* opt)
 
   // The Arm debug fix is a "temporary" fix for issue #3874
   // https://github.com/ponylang/ponyc/issues/3874
-  // Hopefully we get #3874 figured out in a reasonable amount of time.
-  CodeGenOpt::Level opt_level =
-    opt->release ? CodeGenOpt::Aggressive :
+  CodeGenOpt::Level opt_level = opt->release ? CodeGenOpt::Aggressive :
       target_is_arm(opt->triple) ? CodeGenOpt::Default : CodeGenOpt::None;
 
   TargetOptions options;

test/libponyc-run/regression-4369/main.pony

@@ -0,0 +1,26 @@
+use "time"
+
+class TimerPrint is TimerNotify
+  var _count: U64 = 0
+
+  new iso create() =>
+    None
+
+  fun ref apply(timer: Timer, count: U64): Bool =>
+    _count = _count + count
+    _count < 10
+
+  fun ref cancel(timer: Timer) =>
+    None
+
+actor Main
+  new create(env: Env) =>
+    let timers = Timers
+
+    let t1 = Timer(TimerPrint, 500000000, 500000000) // 500 ms
+    let t2 = Timer(TimerPrint, 500000000, 500000000) // 500 ms
+
+    let t1' = t1
+    timers(consume t1)
+    timers.cancel(t1')
+    timers(consume t2)