FIR: Correctly handle nullable/flexible captured types

compiler/fir/analysis-tests/testData/resolve/flexibleCapturedType.kt

@@ -0,0 +1,10 @@
+// FILE: main.kt
+
+fun foo(p: Processor<in CharSequence>, s: String?) {
+    p.process(s)
+}
+
+// FILE: Processor.java
+public interface Processor<T> {
+    boolean process(T t);
+}

compiler/fir/analysis-tests/testData/resolve/flexibleCapturedType.txt

@@ -0,0 +1,4 @@
+FILE: main.kt
+    public final fun foo(p: R|Processor<in kotlin/CharSequence>|, s: R|kotlin/String?|): R|kotlin/Unit| {
+        R|<local>/p|.R|SubstitutionOverride</Processor.process: R|kotlin/Boolean|>|(R|<local>/s|)
+    }