FIR: rename AMBIGUITY to OVERLOAD_RESOLUTION_AMBIGUITY to match FE 1.0

compiler/testData/diagnostics/tests/operatorsOverloading/IteratorAmbiguity.fir.kt

@@ -21,7 +21,7 @@ class MyIterator {
 class MyElement
 
 fun test1(collection: MyCollection) {
-    collection.<!AMBIGUITY!>iterator<!>()
-    <!AMBIGUITY, UNRESOLVED_REFERENCE, UNRESOLVED_REFERENCE!>for (element in collection) {
+    collection.<!OVERLOAD_RESOLUTION_AMBIGUITY!>iterator<!>()
+    <!OVERLOAD_RESOLUTION_AMBIGUITY, UNRESOLVED_REFERENCE, UNRESOLVED_REFERENCE!>for (element in collection) {
     }<!>
 }

compiler/testData/diagnostics/tests/operatorsOverloading/plusAssignOnArray.fir.kt

@@ -16,5 +16,5 @@ fun test() {
     val c = C()
     c[0] += ""
     var c1 = C1()
-    c1[0] <!AMBIGUITY!>+=<!> ""
+    c1[0] <!OVERLOAD_RESOLUTION_AMBIGUITY!>+=<!> ""
 }