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

[via cfe-commits]

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@@ -4084,6 +4084,19 @@ bool Sema::MergeFunctionDecl(FunctionDecl *New, NamedDecl *&OldD, Scope *S,
            diag::note_carries_dependency_missing_first_decl) << 0/*Function*/;
     }
 
+    // SYCL spec 2020
+    //   The first declaration of a function with external linkage must
+    //   specify sycl_external attribute.
+    //   Subsequent declarations may optionally specify this attribute.
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schittir wrote:

Makes sense. Thank you. 

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