[foreign] RFR 8219967: Add clang document introspection API support in jdk.internal.clang module

[Sundararajan Athijegannathan]

Please review.

Rfe: https://bugs.openjdk.java.net/browse/JDK-8219967
Webrev: https://cr.openjdk.java.net/~sundar/8219967/webrev.00/

PS. Piggybacking to make few cleanups:

Made few native methods in jdk.internal.clang private.
Renamed all native methods as foo0 (kind0) pattern as used elsewhere in 
JDK rather than foo1.

I've not added tests because other parts of jdk.internal.clang don't 
seem to have direct tests - those are exercised only via jdk.jextract 
tests. For now, I ran Comment class (which has a test main) on few clang 
headers with documentation and manually inspected the output.

Thanks,
-Sundar