Sort out renderers for diagnostics

- change NAMED to be Renderer<Named> and render Named's name. It was used in
  multiple places with arbitrary arguments, not only Named: change renderers in
  those places to TO_STRING
- add STRING, which is Renderer<String> and renders string itself. Change all
  places where strings were used with either TO_STRING or NAMED to STRING
- change NOT_AN_ANNOTATION_CLASS diagnostic to be reported on descriptor, not
  any string
- change "unused variable/parameter" diagnostics to be reported on
  VariableDescriptor, not Object

idea/testData/diagnosticMessage/assignedButNeverAccessedVariable.kt

@@ -0,0 +1,7 @@
+// !DIAGNOSTICS_NUMBER: 1
+// !DIAGNOSTICS: ASSIGNED_BUT_NEVER_ACCESSED_VARIABLE
+
+fun foo() {
+    var x = "a"
+    x = "b"
+}

idea/testData/diagnosticMessage/assignedButNeverAccessedVariable1.txt

@@ -0,0 +1,2 @@
+<!-- assignedButNeverAccessedVariable1 -->
+Variable 'x' is assigned but never accessed

idea/testData/diagnosticMessage/unusedParameter.kt

@@ -0,0 +1,4 @@
+// !DIAGNOSTICS_NUMBER: 1
+// !DIAGNOSTICS: UNUSED_PARAMETER
+
+fun foo(unused: String) {}

idea/testData/diagnosticMessage/unusedParameter1.txt

@@ -0,0 +1,2 @@
+<!-- unusedParameter1 -->
+Parameter 'unused' is never used

idea/testData/diagnosticMessage/unusedVariable.kt

@@ -0,0 +1,6 @@
+// !DIAGNOSTICS_NUMBER: 1
+// !DIAGNOSTICS: UNUSED_VARIABLE
+
+fun foo() {
+    val x = 42
+}

idea/testData/diagnosticMessage/unusedVariable1.txt

@@ -0,0 +1,2 @@
+<!-- unusedVariable1 -->
+Variable 'x' is never used