Introduce inspection to detect use of Unit as a standalone expression

So #KT-20631 Fixed

idea/testData/inspectionsLocal/redundantUnitExpression/.inspection

@@ -0,0 +1 @@
+org.jetbrains.kotlin.idea.inspections.RedundantUnitExpressionInspection

idea/testData/inspectionsLocal/redundantUnitExpression/notRedundant1.kt

@@ -0,0 +1,4 @@
+// PROBLEM: none
+fun test() {
+    val u = <caret>Unit
+}

idea/testData/inspectionsLocal/redundantUnitExpression/notRedundant2.kt

@@ -0,0 +1,2 @@
+// PROBLEM: none
+fun test() = <caret>Unit

idea/testData/inspectionsLocal/redundantUnitExpression/notRedundant3.kt

@@ -0,0 +1,7 @@
+// PROBLEM: none
+fun test(s: String?) {
+    val x: Any = if (s == null)
+        ""
+    else
+        Unit<caret>
+}

idea/testData/inspectionsLocal/redundantUnitExpression/redundant1.kt

@@ -0,0 +1,3 @@
+fun test() {
+    return <caret>Unit
+}

idea/testData/inspectionsLocal/redundantUnitExpression/redundant1.kt.after

@@ -0,0 +1,3 @@
+fun test() {
+    return
+}

idea/testData/inspectionsLocal/redundantUnitExpression/redundant2.kt

@@ -0,0 +1,5 @@
+fun test() {
+    val f: () -> Unit = {
+        <caret>Unit
+    }
+}

idea/testData/inspectionsLocal/redundantUnitExpression/redundant2.kt.after

@@ -0,0 +1,4 @@
+fun test() {
+    val f: () -> Unit = {
+    }
+}

idea/testData/inspectionsLocal/redundantUnitExpression/redundant3.kt

@@ -0,0 +1,6 @@
+fun test() {
+    val f: () -> Unit = {
+        <caret>Unit
+        Unit
+    }
+}

idea/testData/inspectionsLocal/redundantUnitExpression/redundant3.kt.after

@@ -0,0 +1,5 @@
+fun test() {
+    val f: () -> Unit = {
+        Unit
+    }
+}

idea/testData/inspectionsLocal/redundantUnitExpression/redundant4.kt

@@ -0,0 +1,8 @@
+fun test(s: String?) {
+    val x: Any = if (s == null) {
+        ""
+    }
+    else {
+        <caret>Unit
+    }
+}

idea/testData/inspectionsLocal/redundantUnitExpression/redundant4.kt.after

@@ -0,0 +1,7 @@
+fun test(s: String?) {
+    val x: Any = if (s == null) {
+        ""
+    }
+    else {
+    }
+}

idea/testData/inspectionsLocal/redundantUnitExpression/redundant5.kt

@@ -0,0 +1,7 @@
+// WITH_RUNTIME
+
+fun test() {
+    val x: List<Unit> = listOf(1, 2, 3).map {
+        return@map <caret>Unit
+    }
+}

idea/testData/inspectionsLocal/redundantUnitExpression/redundant5.kt.after

@@ -0,0 +1,7 @@
+// WITH_RUNTIME
+
+fun test() {
+    val x: List<Unit> = listOf(1, 2, 3).map {
+        return@map
+    }
+}