Fix generic signature for SAM adapters and constructors

SAM-related code in codegen was using JavaClassDescriptor directly, which has
an erased signature. Create and use a new abstraction SamType which has a full
generic signature of a type which was used in the SAM construct

compiler/testData/codegen/boxWithJava/sam/adapters/genericSignature.java

@@ -0,0 +1,8 @@
+import java.util.Arrays;
+import java.util.Comparator;
+
+class JavaClass {
+    public static String foo(Comparator<String> comparator) {
+        return Arrays.toString(comparator.getClass().getGenericInterfaces());
+    }
+}