FIR checker: introduce top-level function checker

compiler/testData/diagnostics/tests/Abstract.fir.kt

@@ -22,7 +22,7 @@ package MyPackage
     abstract val e3: Int = 0; get() = a
 
     //methods
-    fun f()
+    <!NON_MEMBER_FUNCTION_NO_BODY!>fun f()<!>
     fun g() {}
     abstract fun h()
     abstract fun j() {}

compiler/testData/diagnostics/tests/DefaultValuesCheckWithoutBody.fir.kt

@@ -6,4 +6,4 @@ abstract class Abst {
     abstract fun foo(x: Int = <!UNRESOLVED_REFERENCE!>y<!>, y: Int = x)
 }
 
-fun extraDiagnostics(x: Int = <!UNRESOLVED_REFERENCE!>y<!>, y: Int)
+<!NON_MEMBER_FUNCTION_NO_BODY!>fun extraDiagnostics(x: Int = <!UNRESOLVED_REFERENCE!>y<!>, y: Int)<!>

compiler/testData/diagnostics/tests/incompleteCode/diagnosticWithSyntaxError/namedFun.fir.kt

@@ -1 +0,0 @@
-fun bar()

compiler/testData/diagnostics/tests/incompleteCode/diagnosticWithSyntaxError/namedFun.kt

@@ -1 +1,2 @@
+// FIR_IDENTICAL
 <!NON_MEMBER_FUNCTION_NO_BODY!>fun bar()<!>

compiler/testData/diagnostics/tests/multiplatform/topLevelFun/implDeclarationWithoutBody.fir.kt

@@ -7,6 +7,6 @@ expect fun foo()
 // MODULE: m2-jvm(m1-common)
 // FILE: jvm.kt
 
-actual fun foo()
+<!NON_MEMBER_FUNCTION_NO_BODY!>actual fun foo()<!>
 
-actual fun bar()
+<!NON_MEMBER_FUNCTION_NO_BODY!>actual fun bar()<!>