RecordType: Ensure &default fields are always re-initialized

This started working partly after the deferral logic introduced with
Zeek 6.0 so this finishes it :-)

testing/btest/Baseline/language.delete-field-set-2/output

@@ -0,0 +1,4 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+default, [v=[1, 2, 3, 100], r=[c=101]]
+after changing, [v=[1, 2, 3, 100, 4711], r=[c=42]]
+after delete, [v=[1, 2, 3, 102], r=[c=103]]

testing/btest/language/delete-field-set.zeek

@@ -1,3 +1,5 @@
+# @TEST-DOC: Test behavior of &default when deleting a record field and subsequently accessing it again.
+#
 # @TEST-EXEC: zeek -b %INPUT >output 2>&1
 # @TEST-EXEC: btest-diff output
 
@@ -14,3 +16,30 @@ delete test$b;
 delete test$c;
 
 print test;
+
+# @TEST-START-NEXT
+global c = 99;
+
+# Helper function that's running as part of R's default construction.
+function seq(): count {
+	++c;
+	return c;
+}
+
+type R: record {
+	c: count &default=seq();
+};
+
+type FooBar: record {
+        v: vector of count &default=vector(1, 2, 3, seq());
+	r: R &default=R();
+};
+
+global test: FooBar;
+print "default", test;
+test$v += 4711;
+test$r$c = 42;
+print "after changing", test;
+delete test$v;
+delete test$r;
+print "after delete", test;