FIR: Properly build nullable suspend function types, and aggregate

modifiers and annotations within KtTypeReference/REFERENCE_TYPE nodes.

compiler/testData/codegen/box/coroutines/nullableSuspendFunctionType.kt

@@ -0,0 +1,25 @@
+// WITH_RUNTIME
+// WITH_COROUTINES
+import helpers.*
+import kotlin.coroutines.*
+import kotlin.coroutines.intrinsics.*
+
+suspend fun suspendHere(): String = suspendCoroutineUninterceptedOrReturn { x ->
+    x.resume("OK")
+    COROUTINE_SUSPENDED
+}
+
+fun builder(c: (suspend () -> Unit)?) {
+    c?.startCoroutine(EmptyContinuation)
+}
+
+fun box(): String {
+    var result = ""
+
+    builder(null)
+    builder {
+        result = suspendHere()
+    }
+
+    return result
+}