[JVM IR] Do not add nullability annotation for functions that return

platform types.

This behavior matches the non-IR backend, which was added in
f16dcdd8a9.

compiler/testData/codegen/bytecodeListing/platformTypes.kt

@@ -0,0 +1,8 @@
+// WITH_RUNTIME
+import java.util.Collections
+
+class PlatformTypes {
+    fun simplyPlatform() = Collections.singletonList("")[0]
+    fun bothNullable() = Collections.emptyList<String>() ?: null
+    fun bothNotNull() = Collections.emptyList<String>()!!
+}

compiler/testData/codegen/bytecodeListing/platformTypes.txt

@@ -0,0 +1,7 @@
+@kotlin.Metadata
+public final class PlatformTypes {
+    public method <init>(): void
+    public final @org.jetbrains.annotations.NotNull method bothNotNull(): java.util.List
+    public final @org.jetbrains.annotations.Nullable method bothNullable(): java.util.List
+    public final method simplyPlatform(): java.lang.String
+}