FIR: Report UNSAFE_INFIX_CALL for all infix-style calls, even if the

candidate is not an infix function.

This mirrors FE 1.0 (see
`AbstractTracingStrategy.reportUnsafeCallOnBinaryExpression()`) and
allows consistent handling for quickfixes on infix calls.

compiler/testData/diagnostics/tests/BinaryCallsOnNullableValues.fir.kt

@@ -6,7 +6,7 @@ fun f(): Unit {
     var x: Int? = 1
     x = null
     x <!UNSAFE_OPERATOR_CALL!>+<!> 1
-    x <!UNSAFE_CALL!>plus<!> 1
+    x <!UNSAFE_INFIX_CALL!>plus<!> 1
     x <!UNSAFE_OPERATOR_CALL!><<!> 1
     x <!UNRESOLVED_REFERENCE!>+=<!> 1