RFR: change langtools tests to use ProblemList instead of @ignore

[Jonathan Gibbons]

In the langtools test suite, the convention has been to use the jtreg 
@ignore mechanism to identify tests which should not be executed for 
some reason.    But other repos in OpenJDK use the more recent and more 
flexible "exclude list" mechanism, as exemplified by files such as 
<repo>/test/ProblemList.txt.

This is to set up the langtools/test/ProblemList.txt file with tests 
that currently contain @ignore in their test description.  The @ignore 
tags are not being removed at this point, to allow for a changeover 
period until everyone is excluding tests using the ProblemList file, 
with an option such as the following:
     -exclude:langtools/test/ProblemList.txt

It is regrettable that the number of ignored tests has been growing. As 
a separate exercise, we need to work on fixing the tests on this list.

JBS: https://bugs.openjdk.java.net/browse/JDK-8152048
Webrev: http://cr.openjdk.java.net/~jjg/8152048/webrev.00/

-- Jon