[clang] [OpenACC] Switch Clang to use the Flang 'appertainment' rules for cla… (PR #135372)

[Erich Keane via cfe-commits]

erichkeane wrote:

@razvanlupusoru @clementval @jefflarkin : This is the place I discussed/promised.

Note I used 'flang' for a stand-in as "after using the Acc.td file in clang", so some of those might not be perfectly accurate.  BUT are ones that need discussion in some way.

We can discuss the changes to OpenACC Spec via email/discussion, as it probably keeps them best organized (and perhaps 1x at a time?).  As you can see, there is plenty to do/discuss :) 

https://github.com/llvm/llvm-project/pull/135372