Continue processing not builder inference stub variables in the type checker during computing common super type

^KT-47148 Fixed

compiler/testData/diagnostics/tests/subtyping/kt47148.kt

@@ -0,0 +1,10 @@
+// WITH_RUNTIME
+
+interface Visitor<T> {
+    fun visit(key: String): T
+}
+val str1: String? = null
+val str2: String? = null
+fun <T> visit(arg: Visitor<T>): T = str1?.<!IMPLICIT_NOTHING_TYPE_ARGUMENT_AGAINST_NOT_NOTHING_EXPECTED_TYPE!>let<!> { return@visit arg.visit(it) }
+    ?: str2?.<!IMPLICIT_NOTHING_TYPE_ARGUMENT_AGAINST_NOT_NOTHING_EXPECTED_TYPE!>let<!> { return@visit arg.visit(it) }
+    ?: error("error")