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

[Erich Keane via cfe-commits]

================
@@ -4084,6 +4084,19 @@ bool Sema::MergeFunctionDecl(FunctionDecl *New, NamedDecl *&OldD, Scope *S,
            diag::note_carries_dependency_missing_first_decl) << 0/*Function*/;
     }
 
+    // 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.
+    const SYCLExternalAttr *SEA = New->getAttr<SYCLExternalAttr>();
+    if (SEA && !Old->hasAttr<SYCLExternalAttr>()) {
----------------
erichkeane wrote:

To be consistent with the rest of our attributes, I'd probably suggest we support adding it up until definition.  We can then do a conformance warning instead of error, but still let it work.

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