RFR: 8149996: TraceLoaderConstraints has been converted to Unified Logging.

[Max Ockner]

Hello again,

Please review this change. TraceLoaderConstraints has been converted to 
Unified Logging and can be accessed using 
'-Xlog:loaderconstraints=info'. TraceLoaderConstraints has been added to 
the logging alias table.

There are 0 lines of output for java -version, and ~10 lines from 
LoaderConstraintsTest.java

bug: https://bugs.openjdk.java.net/browse/JDK-8149996
webrev: http://cr.openjdk.java.net/~mockner/8149996.02/

Testing: jtreg runtime. Added new test which triggers logging for 
loaderconstraints by forcing class unloading. No other references to 
TraceLoaderConstraints found in hotspot/test, jdk/test , or in the 
noncolo tests.

Thanks,
Max