Don't strip debug info in unoptimized builds without -g (#12016)

We used to always run wasm-opt's --strip-debug when -g was not
specified, which would strip out DWARF as well as the Name section.
This changes us to leave it alone. This has no effect on release builds
(-O1+) and no effect on proper debug builds (-O0 -g), but does have
an effect on unoptimized builds (-O0) without -g, which may now contain
DWARF or the Name section now, depending on how clang and wasm-ld
were invoked.

Part of reducing unnecessary work after link, and aligning us more
with what LLVM tools normally do, see
WebAssembly/binaryen#3043

ChangeLog.md

@@ -24,6 +24,10 @@ Current Trunk
   producer's section in debug builds, you can remove it a tool like
   `wasm-opt --strip-producers` (which is what Emscripten still does in release
   builds, as always) or use `llvm-objcopy`.
+- Only strip debug info in release builds + when `-g` is not present. Previously
+  even in an `-O0` build without `-g` we would strip it. This was not documented
+  behavior, and has no effect on program behavior, but may be noticeable
+  if you inspect a build output with `-O0`.
 - Do not remove `__original_main` using `--inline-main`. We used to do this
   so that it didn't show up in stack traces (which could be confusing because
   it is added by the linker - it's not in the source code). But this has had

emcc.py

@@ -601,9 +601,9 @@ def backend_binaryen_passes():
   # hardcoded value in the binaryen pass)
   if shared.Settings.OPT_LEVEL > 0 and shared.Settings.GLOBAL_BASE >= 1024:
     passes += ['--low-memory-unused']
-  if shared.Settings.DEBUG_LEVEL < 3:
-    passes += ['--strip-debug']
   if shared.Settings.OPT_LEVEL > 0:
+    if shared.Settings.DEBUG_LEVEL < 3:
+      passes += ['--strip-debug']
     if not shared.Settings.EMIT_PRODUCERS_SECTION:
       passes += ['--strip-producers']
   if shared.Settings.AUTODEBUG: