[llvm] [clang] [NVPTX] Add 'activemask' builtin and intrinsic support (PR #79768)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> https://bugs.llvm.org/show_bug.cgi?id=35249

Yeah, there's constant issues with convergence analysis. I included one of the tests to try to show that it won't merge with the covergent attribute. Since this is a general issue for all of these things. In the past I usually add instructions like wave level syncs to prevent this. I think there's some extra attributes I can add here to prevent this fully.

I think we need
```
IntrHasSideEffects
```
attribute maybe.

https://github.com/llvm/llvm-project/pull/79768