[LLVM HEAD] Build fails due to memory attribute changes

[TimNN]

Building the compiler fails at LLVM head due to the changes introduced by llvm/llvm-project@304f1d5 ("[IR] Switch everything to use memory attribute"):

cargo:warning=llvm-wrapper/RustWrapper.cpp:217:12: error: no member named 'InaccessibleMemOnly' in 'llvm::Attribute'; did you mean simply 'InaccessibleMemOnly'?
--
  | cargo:warning=    return Attribute::InaccessibleMemOnly;
  | cargo:warning=           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  | cargo:warning=           InaccessibleMemOnly

Based on my (limited) understanding the fix will not be entirely trivial, so opening an issue to track.

Found via our experimental rust + llvm at HEAD bot: https://buildkite.com/llvm-project/rust-llvm-integrate-prototype/builds/14694#01844218-71d4-4b0c-85a3-bf643a088de1

cc @nikic

[TimNN]

I'll give fixing this a go. @rustbot claim

[nikic]

I expect the path of least resistance here would be to export two new functions from our FFI API to create the readonly and inaccessiblememonly attributes. Exposing MemoryEffects to the Rust side is likely not worthwhile at this point.