Change return type quick-fix: Add support of constant expressions

#KT-6260 Fixed
 #KT-6404 Fixed

idea/testData/quickfix/typeMismatch/afterChangeReturnTypeToSpecificNullable.kt

@@ -0,0 +1,6 @@
+// "Change 'foo' function return type to 'Int?'" "true"
+
+fun foo(): Int? {
+    val n: Int? = 1
+    return n
+}

idea/testData/quickfix/typeMismatch/afterConstantTypeMismatch.kt

@@ -0,0 +1,5 @@
+// "Change 'foo' function return type to 'Int'" "true"
+
+fun foo(): Int {
+    return <caret>1
+}

idea/testData/quickfix/typeMismatch/afterMakeReturnTypeNullable.kt

@@ -0,0 +1,5 @@
+// "Change 'foo' function return type to 'String?'" "true"
+
+fun foo(): String? {
+    return null
+}

idea/testData/quickfix/typeMismatch/beforeChangeReturnTypeToSpecificNullable.kt

@@ -0,0 +1,6 @@
+// "Change 'foo' function return type to 'Int?'" "true"
+
+fun foo(): String {
+    val n: Int? = 1
+    return <caret>n
+}

idea/testData/quickfix/typeMismatch/beforeConstantTypeMismatch.kt

@@ -0,0 +1,5 @@
+// "Change 'foo' function return type to 'Int'" "true"
+
+fun foo(): String {
+    return <caret>1
+}

idea/testData/quickfix/typeMismatch/beforeMakeReturnTypeNullable.kt

@@ -0,0 +1,5 @@
+// "Change 'foo' function return type to 'String?'" "true"
+
+fun foo(): String {
+    return <caret>null
+}