Consolidate mono test behavior in debug and release modes

Makes sure that we turn on `debug-symbols` in mono tests, and that this
feature is fully respected in symbol generation, so that output in
release + debug builds are the same.

Closes #4435

crates/compiler/test_mono/src/tests.rs

@@ -143,7 +143,6 @@ fn compiles_to_ir(test_name: &str, src: &str) {
     verify_procedures(test_name, layout_interner, procedures, main_fn_symbol);
 }
 
-#[cfg(debug_assertions)]
 fn verify_procedures<'a>(
     test_name: &str,
     interner: STLayoutInterner<'a>,
@@ -201,19 +200,6 @@ fn verify_procedures<'a>(
     }
 }
 
-// NOTE because the Show instance of module names is different in --release mode,
-// these tests would all fail. In the future, when we do interesting optimizations,
-// we'll likely want some tests for --release too.
-#[cfg(not(debug_assertions))]
-fn verify_procedures(
-    _expected: &str,
-    _interner: STLayoutInterner<'_>,
-    _procedures: MutMap<(Symbol, ProcLayout<'_>), Proc<'_>>,
-    _main_fn_symbol: Symbol,
-) {
-    // Do nothing
-}
-
 #[mono_test]
 fn ir_int_literal() {
     r#"