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/elvisIfUnit.kt

@@ -0,0 +1,9 @@
+// PROBLEM: none
+
+fun nullable() {}
+
+fun bar() {}
+
+fun foo(f: Boolean) {
+    <caret>nullable() ?: if (f) bar()
+}