[foreign] RFR 8234237: Update to LLVM9

[Jorn Vernee]

Hi,

Please review the following:

Bug: https://bugs.openjdk.java.net/browse/JDK-8234237
Webrev: http://cr.openjdk.java.net/~jvernee/panama/webrevs/llvm9/webrev.02

This updates the libclang dependency to LLVM 9.

FTR, since some bugs around Incomplete Arrays were fixed in 9, we can 
also start taking advantage of that. I've done some experiments with 
that and the results are promising, but let's save it for another patch.

Thanks,
Jorn