[Assign plugin] Add a compiler plugin for overloading assign ('=') operator

plugins/assign-plugin/testData/diagnostics/incorrectUsage.kt

@@ -0,0 +1,27 @@
+annotation class ValueContainer
+
+@ValueContainer
+data class StringProperty(var v: String) {
+    fun assign(v: String) {
+        this.v = v
+    }
+    fun assign(v: StringProperty) {
+        this.v = v.get()
+    }
+    fun get(): String = v
+}
+
+data class Task(val input: StringProperty)
+
+fun `should report error if type doesn't match`() {
+    val task = Task(StringProperty("Fail"))
+    <!VAL_REASSIGNMENT!>task.input<!> <!NO_APPLICABLE_ASSIGN_METHOD!>=<!> <!CONSTANT_EXPECTED_TYPE_MISMATCH!>42<!>
+}
+
+fun `should report error if type doesn't match with apply`() {
+    val task = Task(StringProperty("Fail"))
+    task.apply {
+        <!VAL_REASSIGNMENT!>input<!> <!NO_APPLICABLE_ASSIGN_METHOD!>=<!> <!CONSTANT_EXPECTED_TYPE_MISMATCH!>42<!>
+    }
+}
+