Support fun interfaces in kotlinx-metadata

#KT-37421 Fixed

libraries/tools/kotlinp/testData/FunInterface.kt

@@ -0,0 +1,3 @@
+fun interface F {
+    fun String.f(x: Int)
+}

libraries/tools/kotlinp/testData/FunInterface.txt

@@ -0,0 +1,13 @@
+// F.class
+// ------------------------------------------
+public abstract fun interface F : kotlin/Any {
+
+  // signature: f(Ljava/lang/String;I)V
+  public abstract fun kotlin/String.f(x: kotlin/Int): kotlin/Unit
+
+  // module name: test-module
+}
+// META-INF/test-module.kotlin_module
+// ------------------------------------------
+module {
+}