[clang] Add sycl_external attribute (PR #140282)

[Tom Honermann via cfe-commits]

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@@ -408,10 +408,14 @@ class SubjectList<list<AttrSubject> subjects, SubjectDiag diag = WarnDiag,
   string CustomDiag = customDiag;
 }
 
-class LangOpt<string name, code customCode = [{}]> {
+class LangOpt<string name, code customCode = [{}], bit silentlyIgnore = 0> {
   // The language option to test; ignored when custom code is supplied.
   string Name = name;
 
+  // If set to 1, the attribute is accepted but is silently ignored. This is
+  // useful in multi-compilation situations like SYCL.
+  bit SilentlyIgnore = silentlyIgnore;
+
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tahonermann wrote:

None of the proposed changes query this bit (downstream Intel repos have additional code in `GenerateTestExpression()` in `clang/utils/TableGen/ClangAttrEmitter.cpp` that checks it).

I think we should hold off on adding this for now. Or, we should add it via a separate PR that justifies its use based on similar existing utility for CUDA, HIP, HLSL, etc...

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