[nicl] RFR: TranslationUnit class avoid exposure of pointer/address

[Henry Jen]

Hi,

Please review a simple tweak[1] on jdk.internal.clang, added TranslationUnit for libclang related functions and avoid expose long value directly.

There was a FFI version of clang binding using jextract directly, and converting long and Pointer is sensitive and need to be careful. Also TranslationUnit is proper representation for libclang CXTranslationUnit.

[1] http://cr.openjdk.java.net/~henryjen/panama/clangTU/0/webrev/

Cheers,
Henry