Improve generated names for anonymous classes

In order of preference, the generated name will be:

new class extends ParentClass {};
// -> ParentClass@anonymous
n

Improve generated names for anonymous classes

In order of preference, the generated name will be:

new class extends ParentClass {};
// -> ParentClass@anonymous
new class implements FirstInterface, SecondInterface {};
// -> FirstInterface@anonymous
new class {};
// -> class@anonymous

This is intended to display a more useful class name in error messages
and stack traces, and thus make debugging easier.

Closes GH-5153.

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