[clang] Add sycl_external attribute (PR #140282)

[Tom Honermann via cfe-commits]

================
@@ -62,6 +62,7 @@ class SemaSYCL : public SemaBase {
                                        ParsedType ParsedTy);
 
   void handleKernelAttr(Decl *D, const ParsedAttr &AL);
+  void handleSYCLExternalAttr(Decl *D, const ParsedAttr &AL);
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tahonermann wrote:

Since this function is a member of the `SemaSYCL` class, I think we can drop "SYCL" from its name. This is consistent with what was done for the other attribute handlers.
```suggestion
  void handleExternalAttr(Decl *D, const ParsedAttr &AL);
```

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