[FIR] Add diagnostic CONFLICTING_OVERLOADS & REDECLARATION

compiler/tests-spec/testData/diagnostics/linked/overload-resolution/choosing-the-most-specific-candidate-from-the-overload-candidate-set/algorithm-of-msc-selection/p-14/neg/1.1.fir.kt

@@ -18,7 +18,7 @@ class Case2() {
     fun foo(vararg x: Int?): Unit = TODO() // (1.2)
 
     fun case(){
-        foo(1, 1)
+        <!AMBIGUITY!>foo<!>(1, 1)
     }
 }
 // TESTCASE NUMBER: 3
@@ -27,7 +27,7 @@ class Case3() {
     fun foo(vararg x: Int?): Unit = TODO() // (1.2)
 
     fun case(){
-        foo(1, 1)
+        <!AMBIGUITY!>foo<!>(1, 1)
     }
 }