Receiver of a safe call is not null inside the call also for extension functions + a pair of tests #KT-9033 Fixed

compiler/testData/diagnostics/tests/smartCasts/safecalls/extensionCall.kt

@@ -0,0 +1,6 @@
+fun String.bar(s: String) = s
+
+fun foo(s: String?) {
+    s?.bar(<!DEBUG_INFO_SMARTCAST!>s<!>)
+    s?.charAt(<!DEBUG_INFO_SMARTCAST!>s<!>.length())
+}

compiler/testData/diagnostics/tests/smartCasts/safecalls/extensionCall.txt

@@ -0,0 +1,4 @@
+package
+
+public fun foo(/*0*/ s: kotlin.String?): kotlin.Unit
+public fun kotlin.String.bar(/*0*/ s: kotlin.String): kotlin.String