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

[Tom Honermann via cfe-commits]

================
@@ -0,0 +1,54 @@
+// RUN: %clang_cc1 -fsycl-is-device -fsyntax-only -verify %s
+
+// Semantic tests for sycl_external attribute
+
+[[clang::sycl_external]] // expected-error {{'sycl_external' can only be applied to functions with external linkage}}
+static void func1() {}
+
+namespace {
+  [[clang::sycl_external]] // expected-error {{'sycl_external' can only be applied to functions with external linkage}}
+  void func2() {}
+
+  struct UnnX {};
+}
+
+[[clang::sycl_external]] // expected-error {{'sycl_external' can only be applied to functions with external linkage}}
+  void func4(UnnX) {}
+
+// FIXME: The first declaration of a function is required to have the attribute.
+// The attribute may be optionally present on subsequent declarations
+int foo(int c);
+
+[[clang::sycl_external]] void foo();
+
+class C {
+  [[clang::sycl_external]] void member();
+};
+
+[[clang::sycl_external]] int main() // expected-error {{'sycl_external' cannot be applied to main function}}
+{
+    return 0;
----------------
tahonermann wrote:

Keep indentation consistent.
```suggestion
  return 0;
```

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