Introduce "use expression body" inspection #KT-16063 Fixed

Converted from the relevant intention
Reported cases: one-liners, whens
Also, more exact caret detection in local inspection tests

idea/testData/inspectionsLocal/useExpressionBody/convertToExpressionBody/keepComments/4.kt

@@ -0,0 +1,4 @@
+fun getText(): String {
+    /* let's return xxx */
+    <caret>return "xxx" /* TODO */
+}