Class.getConstructor() performance regression

Additional security checks have been added to various methods of java.lang.Class, which caused a performance regression. These checks only need to be performed when a SecurityManager is enabled, but the code is structured in a way that additional methods are called even when a SecurityManager is not enabled, such as the relatively expensive Reflection.getCallerClass method.

The fix is to restructure the calls to checkMemberAccess and checkPackageAccess so that they are only invoked when a SecurityManager is enabled.