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

[Tom Honermann 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.
----------------
tahonermann wrote:

It is helpful to quote the specification verbatim so that the intent is understood from the quoted context in the event that the specification changes.
```suggestion
    // SYCL 2020 section 5.10.1, "SYCL functions and member functions linkage":
    //   When a function is declared with SYCL_EXTERNAL, that macro must be
    //   used on the first declaration of that function in the translation unit.
    //   Redeclarations of the function in the same translation unit may
    //   optionally use SYCL_EXTERNAL, but this is not required.
```

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