8350514: Refactor MandatoryWarningHandler to support dynamic verbosity

[archiecobbs]

This PR is a sub-task split off from JDK-8224228, which seeks to add @SuppressWarnings support for lexical features.

The MandatoryWarningHandler is used by the preview, deprecation, removal, and unchecked mandatory warnings. The constructor takes a boolean verbose flag which is documented to "Specify whether or not detailed messages about individual instances should be given, or whether an aggregate message should be generated at the end of the compilation."

The problem is that this flag doesn't really make sense for warnings that are suppressible via @SuppressWarnings: for such warnings, what we actually want to do is trigger the aggregate message at the end of compilation if and only if there were any warnings that were not suppressed by @SuppressWarnings but were suppressed because the corresponding lint category was not enabled, either because it wasn't enabled by default, or due to an explicit -Xlint:-foo flag.

Currently, we get that same net result, because preview is not suppressible via @SuppressWarnings, and for the other three categories there is logic around the calls to MandatoryWarningHandler.report() to ensure the right thing happens.

It would be simpler and more straightforward for the users of MandatoryWarningHandler to just pass along the currently applicable Lint instance and let MandatoryWarningHandler keep track of whether to generate the aggregate message at the end of the compilation.

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• JDK-8350514: Refactor MandatoryWarningHandler to support dynamic verbosity (Sub-task - P4)

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