FIR: keep nullability when serializing suspend function types

No clue where to put the test for that since diagnostic tests, even
multi-module ones, never touch the metadata serializer. So the test is
a bytecode text one pretending to be about nullability annotations even
though it also affects what resolution in another module will do.

compiler/fir/fir2ir/testData/codegen/bytecodeListing/nullabilityAnnotations/suspendFunction.kt

@@ -0,0 +1,11 @@
+// MODULE: lib
+// FILE: 1.kt
+interface I {
+    fun foo(x: (suspend () -> Unit)?): (suspend () -> Unit)?
+}
+
+// MODULE: main(lib)
+// FILE: 2.kt
+class C : I {
+    override fun foo(x: (suspend () -> Unit)?) = x
+}

compiler/fir/fir2ir/testData/codegen/bytecodeListing/nullabilityAnnotations/suspendFunction.txt

@@ -0,0 +1,13 @@
+Module: lib
+@kotlin.Metadata
+public interface I {
+    // source: '1.kt'
+    public abstract @org.jetbrains.annotations.Nullable method foo(@org.jetbrains.annotations.Nullable p0: kotlin.jvm.functions.Function1): kotlin.jvm.functions.Function1
+}
+Module: main
+@kotlin.Metadata
+public final class C {
+    // source: '2.kt'
+    public method <init>(): void
+    public @org.jetbrains.annotations.Nullable method foo(@org.jetbrains.annotations.Nullable p0: kotlin.jvm.functions.Function1): kotlin.jvm.functions.Function1
+}