RFR: 8149995: TraceClassLoadingPreorder has been converted to Unified Logging.

[Max Ockner]

Hi,

Please review this small Unified Logging change. 
TraceClassLoadingPreorder has been converted to UL at the Debug level. 
Debug level was chosen since there are 342 lines of output for java 
-version.

bug: https://bugs.openjdk.java.net/browse/JDK-8149995
webrev: http://cr.openjdk.java.net/~mockner/8149995/

Tested with jtreg runtime tests.
No existing tests reference TraceClassLoadingPreorder from hotspot/test 
or jdk/test.

Thanks,
Max