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

[Tom Honermann via cfe-commits]

================
@@ -476,6 +476,41 @@ The SYCL kernel in the previous code sample meets these expectations.
   }];
 }
 
+def SYCLExternalDocs : Documentation {
+  let Category = DocCatFunction;
+  let Heading = "sycl_external";
+  let Content = [{
+The ``sycl_external`` attribute indicates that a function defined in another
+translation unit may be called by a device function defined in the current
+translation unit or, if defined in the current translation unit, the function
+may be called by device functions defined in other translation units.
+The attribute is intended for use in the implementation of the ``SYCL_EXTERNAL``
+macro as specified in section 5.10.1, "SYCL functions and member functions
+linkage", of the SYCL 2020 specification.
+The attribute only appertains to functions and only those that meet the
+following requirements.
+* Has external linkage.
+* Is not explicitly defined as deleted (the function may be an explicitly
+  defaulted function that is defined as deleted).
+The attribute shall be present on the first declaration of a function and
+may optionally be present on subsequent declarations.
+When compiling for a SYCL device target that does not support the generic
+address space, the function shall not specify a raw pointer or reference type
+as the return type or as a parameter type.
+See section 5.9, "Address-space deduction", of the SYCL 2020 specification.
+The following examples demonstrate the use of this attribute:
----------------
tahonermann wrote:

I think blank lines are required between paragraphs and lists.
```suggestion

The attribute only appertains to functions and only those that meet the
following requirements.

* Has external linkage.
* Is not explicitly defined as deleted (the function may be an explicitly
  defaulted function that is defined as deleted).

The attribute shall be present on the first declaration of a function and
may optionally be present on subsequent declarations.

When compiling for a SYCL device target that does not support the generic
address space, the function shall not specify a raw pointer or reference type
as the return type or as a parameter type.
See section 5.9, "Address-space deduction", of the SYCL 2020 specification.

The following examples demonstrate the use of this attribute:
```

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