Do not suggest "Redundant Unit return type" for Nothing-typed expression

So #KT-20261 Fixed

idea/testData/inspections/redundantUnitReturnType/Test.kt

@@ -7,4 +7,10 @@ fun bar(): Unit {
 
 fun baz(): Unit = println("ok")
 
-fun f1(): Int = 1
+fun f1(): Int = 1
+
+fun f2(): Unit {
+    throw UnsupportedOperationException("")
+}
+
+fun f3(): Unit = throw UnsupportedOperationException("")

idea/testData/inspections/redundantUnitReturnType/inspectionData/expected.xml

@@ -23,6 +23,14 @@
         <problem_class severity="WARNING" attribute_key="NOT_USED_ELEMENT_ATTRIBUTES">Redundant 'Unit' return type</problem_class>
         <description>Redundant Unit return type</description>
     </problem>
+    <problem>
+        <file>Test.kt</file>
+        <line>12</line>
+        <module>light_idea_test_case</module>
+        <entry_point TYPE="file" FQNAME="temp:///src/Test.kt" />
+        <problem_class severity="WARNING" attribute_key="NOT_USED_ELEMENT_ATTRIBUTES">Redundant 'Unit' return type</problem_class>
+        <description>Redundant Unit return type</description>
+    </problem>
     <problem>
         <file>WithLambda.kt</file>
         <line>11</line>