[foreign] Supporting different versions of LLVM

[Jorn Vernee]

Hi,

Over the weekend I have been looking into a fix for: 
https://bugs.openjdk.java.net/browse/JDK-8223031

The underlying problem is that the behaviour of one of the libclang 
function, namely `clang_Cursor_isAnonymous`, was changed to do something 
very different [1], and I haven't been able to find a good workaround 
that covers all needed cases.

So, instead I've submitted a patch to clang to add a new function that 
has the old behaviour [2], which is currently under review and seems to 
be a-go for commit soon. With a small patch on our side we can make our 
code work with the new function [3].

While I was at it I also submitted a patch for the incomplete array 
problem I worked around before [4].

Hopefully these patches will make it into the next LLVM release. But, it 
brings up the question of which LLVM versions should be supported by us. 
Currently I'm locally using version 7.0.1 which works fine, and version 
8.0.0 is definitely not supported due to the `clang_Cursor_isAnonymous` 
change. Once a version is released that has [2], we can support that 
version as well, and with a version that supports [4] we can also remove 
the incomplete array problem workaround (which is pretty extensive). 
Though the latter would mean dropping support for earlier LLVM versions 
as well. I think as long as there's a binary distribution available this 
will not be a problem. We can update the lib-clang autoconf script to 
check the LLVM version, and warn about any that are not supported.

How does that sound?

Thanks,
Jorn

[1] : https://reviews.llvm.org/D54996
[2] : https://reviews.llvm.org/D61232
[3] : 
http://cr.openjdk.java.net/~jvernee/panama/webrevs/8223031/webrev.00/
[4] : https://reviews.llvm.org/D61239