[FIR] Check for ConeErrorType in doUnify to prevent endless recursion

^KT-64625 Fixed

compiler/testData/diagnostics/tests/exceptions/stackOverflowOnDoUnify.kt

@@ -0,0 +1,8 @@
+// FIR_IDENTICAL
+// ISSUE: KT-64625
+// !DIAGNOSTICS: -DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE
+
+interface A<T1, T2>
+interface B<X> : A<X, <!UNRESOLVED_REFERENCE!>T<!>?>
+
+fun f(x: A<Int, <!UNRESOLVED_REFERENCE!>K<!>?>) = x as B