[clang] Add sycl_external attribute (PR #140282)

[Tom Honermann via cfe-commits]

================
@@ -12926,6 +12929,10 @@ bool ASTContext::DeclMustBeEmitted(const Decl *D) {
 
     // FIXME: Functions declared with SYCL_EXTERNAL are required during
     // device compilation.
+    // Functions definitions with sycl_external attribute are required during
+    // device compilation.
----------------
tahonermann wrote:

```suggestion
    // Function definitions with the sycl_external attribute are required during
    // device compilation regardless of whether they are reachable from a SYCL
    // kernel.
```

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