[clang] [spirv][amdgpu] Set atomic size in the clang target info (PR #128569)

[Jon Chesterfield via cfe-commits]

https://github.com/JonChesterfield created https://github.com/llvm/llvm-project/pull/128569

Problem identified by Joseph. The openmp device runtime uses __scoped_atomic_load_n and similar which presently hit 

```
error: large atomic operation may incur significant performance
      penalty; the access size (4 bytes) exceeds the max lock-free size (0 bytes) [-Werror,-Watomic-alignment]
```

This is because the spirv class doesn't set the corresponding field. The base does, but only if there's a host toolchain, which there isn't.

>From a703b8540718e48f5d649e0d139a186890161809 Mon Sep 17 00:00:00 2001
From: Jon Chesterfield <jonathanchesterfield at gmail.com>
Date: Mon, 24 Feb 2025 20:39:35 +0000
Subject: [PATCH] [spirv][amdgpu] Set atomic size in the clang target info

---
 clang/lib/Basic/Targets/SPIR.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/clang/lib/Basic/Targets/SPIR.h b/clang/lib/Basic/Targets/SPIR.h
index 61f9ef7e3e361..610efa1fe00d9 100644
--- a/clang/lib/Basic/Targets/SPIR.h
+++ b/clang/lib/Basic/Targets/SPIR.h
@@ -399,6 +399,8 @@ class LLVM_LIBRARY_VISIBILITY SPIRV64AMDGCNTargetInfo final
     HasLegalHalfType = true;
     HasFloat16 = true;
     HalfArgsAndReturns = true;
+
+    MaxAtomicPromoteWidth = MaxAtomicInlineWidth = 64;
   }
 
   bool hasBFloat16Type() const override { return true; }