Fix few issues with open-source Intel LLVM (Clang 13)

Fix few issues with open-source Intel LLVM (Clang 13)

- Add -Wno-reserved-identifier to avoid false-positive with _real
  user-defined literal (https://bugs.llvm.org/show_bug.cgi?id=50644).
- Change the alignment-checking code to avoid "warning: performing
  pointer subtraction with a null pointer may have undefined behavior".
- Update some comments about SYCL version quirks.

There are some deprecation warnings related to SYCL naming being brought
into agreement with the final SYCL2020 standard, but those will be fixed
once they appear in any official release.