Fix issue #1819 incorrect enum value name generation with leading digits

The first character of an enum value name is checked whether it complies with isJavaIdentifierStart, if not an underscore prefix will be added to that name.

Author: TheMinefighter

jaxb-ri/xjc/src/main/java/com/sun/tools/xjc/reader/xmlschema/SimpleTypeBuilder.java

@@ -674,6 +674,8 @@ private List<CEnumConstant> buildCEnumConstants(XSRestrictionSimpleType type, bo
 
                 if(name==null) {
                     StringBuilder sb = new StringBuilder();
+                    if (facetValue.length()>0 && !Character.isJavaIdentifierStart(facetValue.charAt(0)))
+                        sb.append('_');
                     for( int i=0; i<facetValue.length(); i++) {
                         char ch = facetValue.charAt(i);
                         if(Character.isJavaIdentifierPart(ch))