[FIR] Report INAPPLICABLE_CANDIDATE or more specific diagnostic for callable references

... instead of just UNRESOLVED_REFERENCE when something went wrong
during resolution.

#KT-59401 related

compiler/testData/diagnostics/tests/callableReference/bound/reservedExpressionSyntax2.fir.kt

@@ -4,4 +4,4 @@ package test
 fun nullableFun(): Int? = null
 fun Int.foo() {}
 
-val test1 = nullableFun()?::<!UNRESOLVED_REFERENCE!>foo<!>
+val test1 = nullableFun()?::<!UNSAFE_CALL!>foo<!>