FIR: add UnsafeCall resolution diagnostics

Previously unsafe call is reported as part of InapplicableWrongReceiver.
This makes it difficult for the downstream checkers to report different
diagnostics.

compiler/testData/diagnostics/tests/generics/nullability/expressionsBoundsViolation.fir.kt

@@ -13,7 +13,7 @@ fun <F : String?> bar(x: F) {
     foo1(<!ARGUMENT_TYPE_MISMATCH!>x<!>)
     foo1<F>(<!ARGUMENT_TYPE_MISMATCH!>x<!>)
 
-    x.<!INAPPLICABLE_CANDIDATE!>foo2<!>()
+    x<!UNSAFE_CALL!>.<!>foo2()
     x.<!INAPPLICABLE_CANDIDATE!>foo2<!><F>()
 }