FIR Completion: Fix completion in function calls (foo.bar<caret>())

In such expressions, `KtCallExpression` wraps `KtSimpleNameExpression`,
so `getQualifiedExpressionForSelector()` returns `null`

Also, enable tests that are fixed by this

idea/idea-completion/testData/basic/common/primitiveCompletion/extensionRecompletion.kt

@@ -0,0 +1,13 @@
+// FIR_COMPARISON
+
+class LocalClass
+
+fun LocalClass.ext(action: () -> Unit) {}
+fun LocalClass.extAnother(action: () -> Unit) {}
+
+fun usage(l: LocalClass) {
+    l.ext<caret> {}
+}
+
+// EXIST: ext
+// EXIST: extAnother