GH-16067: prevent invalid `abstract` during compilation of methods (GH-16069)

For classes that are not declared `abstract`, produce a compiler error for any
`abstract` methods. For anony

GH-16067: prevent invalid `abstract` during compilation of methods (GH-16069)

For classes that are not declared `abstract`, produce a compiler error for any
`abstract` methods. For anonymous classes, since they cannot be made abstract,
the error message is slightly different.

Co-authored-by: Ilija Tovilo <ilija.tovilo@me.com>

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