FIR: rename HIDDEN to INVISIBLE_REFERENCE

Some of them should be INVISIBLE_MEMBER though

compiler/testData/diagnostics/tests/variance/privateToThis/FunctionCall.fir.kt

@@ -33,5 +33,5 @@ class Test<in I> {
 }
 
 fun <I> test(t: Test<I>) {
-    t.apply(t.<!HIDDEN!>foo<!>())
+    t.apply(t.<!INVISIBLE_REFERENCE!>foo<!>())
 }

compiler/testData/diagnostics/tests/variance/privateToThis/GetVal.fir.kt

@@ -33,5 +33,5 @@ class Test<in I, out O> {
 }
 
 fun <I, O> test(t: Test<I, O>) {
-    t.apply(t.<!HIDDEN!>i<!>)
+    t.apply(t.<!INVISIBLE_REFERENCE!>i<!>)
 }

compiler/testData/diagnostics/tests/variance/privateToThis/SetVar.fir.kt

@@ -33,5 +33,5 @@ class Test<in I, out O> {
 }
 
 fun <I, O> test(t: Test<I, O>) {
-    t.<!HIDDEN!>i<!> = getT()
+    t.<!INVISIBLE_REFERENCE!>i<!> = getT()
 }