RFR: 7903270: Util classes should be final and have a private constructor [v4]

[Per Minborg]

On Tue, 30 Aug 2022 08:55:44 GMT, Per Minborg <duke at openjdk.org> wrote:

>> This PR makes some of the classes and fields `final`. Also, a general cleanup of the `RuntimeHelper` class is proposed.
>> 
>> There are some files that are "overhidden" (by attempting to "override"/hide static methods). This will be fixed in a separate PR and so these classes are not affected in this PR.
>
> Per Minborg has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add option to show generating class

src/main/java/org/openjdk/jextract/impl/ClassSourceBuilder.java line 202:

> 200:             assert packageName().indexOf('/') == -1 : "package name invalid: " + packageName();
> 201:             append("// Generated by jextract");
> 202:             if (SHOW_GENERATING_CLASS) {

This is very useful when figuring out how a class is generated.

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PR: https://git.openjdk.org/jextract/pull/65