FIC: introduce language feature, modifiers checks and basic tests

compiler/testData/codegen/box/inlineClasses/funInterface/basicFunInterface.kt

@@ -0,0 +1,15 @@
+// !LANGUAGE: +FunctionInterfaceConversion
+
+fun interface Foo {
+    fun invoke(): String
+}
+
+class A : Foo {
+    override fun invoke(): String {
+        return "OK"
+    }
+}
+
+fun box(): String {
+    return A().invoke()
+}

compiler/testData/diagnostics/tests/funInterface/basicFunInterface.kt

@@ -0,0 +1,5 @@
+// !LANGUAGE: +FunctionInterfaceConversion
+
+fun interface Foo {
+    fun invoke()
+}

compiler/testData/diagnostics/tests/funInterface/basicFunInterface.txt

@@ -0,0 +1,8 @@
+package
+
+public interface Foo {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract fun invoke(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/testData/diagnostics/tests/funInterface/basicFunInterfaceDisabled.kt

@@ -0,0 +1,5 @@
+// !LANGUAGE: -FunctionInterfaceConversion
+
+<!UNSUPPORTED_FEATURE!>fun<!> interface Foo {
+    fun invoke()
+}

compiler/testData/diagnostics/tests/funInterface/basicFunInterfaceDisabled.txt

@@ -0,0 +1,8 @@
+package
+
+public interface Foo {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract fun invoke(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}