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

[Erich Keane via cfe-commits]

================
@@ -12937,6 +12937,10 @@ bool ASTContext::DeclMustBeEmitted(const Decl *D) {
   if (D->hasAttr<WeakRefAttr>())
     return false;
 
+  if (LangOpts.SYCLIsDevice && !D->hasAttr<SYCLKernelEntryPointAttr>() &&
+      !D->hasAttr<SYCLExternalAttr>())
----------------
erichkeane wrote:

Hmmm... this skips quite a bit which causes obvious discomfort.  

There's quite a bit inside of the function itself that get skipped too, which is a little concerning, particularly with us all-over-the-place.

IMO, I think a better formulation for the purposes of readability of this would to better integrate these in this function.

~12956 (though perhaps higher than 12950?):
```
// Some comment about how other attributes/etc are all irrelevant for the purposes of SYCL.
if (LangOpts.SYCLIsDevice) 
      return FD->hasAttr<SYCLKernelEntryPointAttr>() || FD->hasAttr<SYCLExternalAttr>();
```

Then around 12990:
```
// Some comment about how global variables should never be emitted
if(LangOpts.SYCLIsDevice) return false;
```

That way this requires MUCH less tea-leaf reading of the rest of the function to figure out when/why we're excluding these.

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