Introduce "double negation" inspection #KT-4748 Fixed

idea/testData/inspectionsLocal/doubleNegation/.inspection

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

idea/testData/inspectionsLocal/doubleNegation/function.kt

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+fun foo() {
+    val b = !!<caret>"".equals("")
+}

idea/testData/inspectionsLocal/doubleNegation/function.kt.after

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+fun foo() {
+    val b = "".equals("")
+}

idea/testData/inspectionsLocal/doubleNegation/invalid.kt

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+// PROBLEM: none
+operator fun Int.not() = this * -1
+fun foo() {
+    val c = !!<caret>1
+}

idea/testData/inspectionsLocal/doubleNegation/parenthesized.kt

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+fun foo() {
+    val b = !(!<caret>true)
+}

idea/testData/inspectionsLocal/doubleNegation/parenthesized.kt.after

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+fun foo() {
+    val b = true
+}

idea/testData/inspectionsLocal/doubleNegation/simple.kt

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+fun foo() {
+    val b = !!<caret>true
+}

idea/testData/inspectionsLocal/doubleNegation/simple.kt.after

@@ -0,0 +1,3 @@
+fun foo() {
+    val b = true
+}