[SYCL] Add __SYCL_INLINE attribute to cl namespace in library sources

[bader]

This change aligns class/functions definitions with the declarations in the headers.

[rolandschulz]

This doesn't work if the user sets __SYCL_DISABLE_NAMESPACE_INLINE__, does it? Then things aren't aligned anymore because the library is built different than the user code. Should we remove __SYCL_DISABLE_NAMESPACE_INLINE__?

[bader]

This doesn't work if the user sets __SYCL_DISABLE_NAMESPACE_INLINE__, does it? Then things aren't aligned anymore because the library is built different than the user code. Should we remove __SYCL_DISABLE_NAMESPACE_INLINE__?

According to my understanding it works. We need to align declarations with definitions at library compilation time to avoid compiler warnings, which are treated as errors. When compiled, these symbols use exposed as fully qualified to the linker, which do not have inline namespace concept - it's used only by the compiler front-end for symbol resolution at compile time.
Let me know if you still have concerns wrt this patch.

[rolandschulz]

Let me know if you still have concerns wrt this patch.

No concerns left. I didn't realize that the mangled name is identical between inline and non-inline namespace. Given that this happens to be the case, it should be fine.