Support fun interfaces in kotlinx-metadata

#KT-37421 Fixed

compiler/testData/loadJava/compiledKotlin/class/FunInterface.kt

@@ -0,0 +1,5 @@
+package test
+
+fun interface F {
+    fun String.f(x: Int)
+}

compiler/testData/loadJava/compiledKotlin/class/FunInterface.txt

@@ -0,0 +1,5 @@
+package test
+
+public fun interface F {
+    public abstract fun kotlin.String.f(/*0*/ x: kotlin.Int): kotlin.Unit
+}