FIR: Support exhaustive whens on subjects of intersection type

compiler/testData/diagnostics/tests/when/intersectionExhaustivenessSimple.kt

@@ -0,0 +1,21 @@
+// SKIP_TXT
+// FIR_IDENTICAL
+// !CHECK_TYPE
+
+sealed class KtClassifierSymbol
+
+interface KtNamedSymbol
+
+abstract class KtTypeParameterSymbol : KtClassifierSymbol() {}
+
+sealed class KtClassLikeSymbol : KtClassifierSymbol() {}
+
+fun foo(symbol: KtClassifierSymbol) {
+    if (symbol !is KtNamedSymbol) return
+    val x = when (symbol) {
+        is KtClassLikeSymbol -> "1"
+        is KtTypeParameterSymbol -> "2"
+    }
+
+    x checkType { _<String>() }
+}