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

[Tom Honermann via cfe-commits]

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@@ -12251,6 +12264,9 @@ bool Sema::CheckFunctionDeclaration(Scope *S, FunctionDecl *NewFD,
   if (NewFD->hasAttr<SYCLKernelEntryPointAttr>())
     SYCL().CheckSYCLEntryPointFunctionDecl(NewFD);
 
+  if (NewFD->hasAttr<SYCLExternalAttr>())
+    SYCL().CheckSYCLExternalFunctionDecl(NewFD);
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tahonermann wrote:

Linkage, or rather, external visibility, which is what we actually check, can depend on other attributes like `VisibilityAttr`. Checking here ensures that all attributes have been processed and therefore avoids visitation ordering issues.

The attribute is allowed on function templates and is automatically inherited by (implicit and explicit) instantiations (and explicit specializations which is incorrect according to the C++ standard). I don't think there is anything to do to handle instantiation.

We do have a testing gap to address yet though. We have good tests for diagnostics, but are missing a test to validate which symbols are actually emitted. We'll ensure that test exercises function templates.

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