[Wasm] Fix unit issues

- Materialize unit when its value is actually needed.
- Special-case Unit_getInstance return type at codegen. It should be a
  proper Unit object instead of a "void"

compiler/testData/codegen/box/unit/nullableUnitInWhen3.kt

@@ -1,5 +1,3 @@
-// DONT_TARGET_EXACT_BACKEND: WASM
-// WASM_MUTE_REASON: UNIT_ISSUES
 fun foo() {}
 
 fun box(): String {