[clang] [clang][SYCL] Add sycl_external attribute and restrict emitting device code (PR #140282)

[Tom Honermann via cfe-commits]

================
@@ -1641,6 +1641,13 @@ def DeviceKernel : DeclOrTypeAttr {
 }];
 }
 
+def SYCLExternal : InheritableAttr {
+  let Spellings = [Clang<"sycl_external">];
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tahonermann wrote:

We use the `Clang` spelling for `sycl_kernel_entry_point`, what would be the reason for doing differently here? The attribute is an implementation detail used to provide the `SYCL_EXTERNAL` functionality, so shouldn't be directly written by SYCL programmers. Are you suggesting excluding the `GNU` spelling?

SYCL is only relevant for C++, so the `C23` spelling wouldn't be desired.

If we switch to (only) the `CXX11` spelling, I think the `clang` namespace should be retained.
```suggestion
  let Spellings = [CXX11<"clang", "sycl_external">];
```

https://github.com/llvm/llvm-project/pull/140282