[NFC][SYCL] Use "inline constexpr" unconditionally

[aelovikov-intel]

C++17 is minimally supported standard, so it is always available to us.

[aelovikov-intel]

Unresolved Tests (5):
  SYCL :: Assert/assert_in_kernels.cpp
  SYCL :: Assert/assert_in_multiple_tus.cpp
  SYCL :: Assert/assert_in_multiple_tus_one_ndebug.cpp
  SYCL :: Assert/assert_in_one_kernel.cpp
  SYCL :: Assert/assert_in_simultaneously_multiple_tus_one_ndebug.cpp

are known - #7634.

@intel/llvm-gatekeepers , this PR is ready.