Java object factory: initialize AbstractStringBuilder-derived types correctly

These are currently initialized as if they are directly derived from Object, which causes a crash due to the
type inconsistency between StringBuilder { AbstractStringBuilder { Object { } } } and StringBuilder { Object { } }

With this change they are initialised more like normal types, which has the side-effect that any fields they possess
that are *not* special-cased by `initialize_nondet_string_fields` are nondet-initialized as for any other type.

I add a test verifying this new behaviour, and a simpler test checking that Builder and Buffer are usable as
nondet arguments at all.

Author: smowton

jbmc/regression/jbmc-strings/NondetStringBuilderAndBuffer/Test.class

@@ -0,0 +1,7 @@
+public class Test {
+
+  public static void testme(StringBuilder builder, StringBuffer buffer) {
+
+  }
+
+}

jbmc/regression/jbmc-strings/NondetStringBuilderAndBuffer/Test.java

@@ -0,0 +1,5 @@
+package java.lang;
+
+public class AbstractStringBuilder implements CharSequence {
+
+}

jbmc/regression/jbmc-strings/NondetStringBuilderAndBuffer/java/lang/AbstractStringBuilder.class

@@ -0,0 +1,5 @@
+package java.lang;
+
+interface CharSequence {
+
+}

jbmc/regression/jbmc-strings/NondetStringBuilderAndBuffer/java/lang/AbstractStringBuilder.java

@@ -0,0 +1,5 @@
+package java.lang;
+
+public class String implements CharSequence {
+
+}

jbmc/regression/jbmc-strings/NondetStringBuilderAndBuffer/java/lang/CharSequence.class

@@ -0,0 +1,5 @@
+package java.lang;
+
+public class StringBuffer extends AbstractStringBuilder {
+
+}