Recognize the RHS of assignments as the JSDoc target expression

Fixes #6552

tests/cases/fourslash/getJavaScriptCompletions18.ts

@@ -0,0 +1,21 @@
+/// <reference path="fourslash.ts" />
+
+// @allowNonTsExtensions: true
+// @Filename: file.js
+//// /**
+////   * @param {number} a
+////   * @param {string} b
+//// */
+//// exports.foo = function(a, b) {
+//// 	a/*a*/;
+//// 	b/*b*/
+//// };
+
+goTo.marker('a');
+edit.insert('.');
+verify.completionListContains('toFixed', undefined, undefined, 'method');
+
+
+goTo.marker('b');
+edit.insert('.');
+verify.completionListContains('substr', undefined, undefined, 'method');