[SYCL] Improve SYCL library versioning mechanism

Currently users can check which SYCL library version they use with __SYCL_COMPILER_VERSION macro. This macro is set in CL/sycl/version.hpp file to a date when the library is built.
However, there are some disadvantages of this approach.
At first, the name of this variable is not accurate. It isn't compiler version, but library version.
At second, building time may not correlate with the last changes in the library. The same source code may be built on the 1st January of 2020 and on the 31st December of 2030.
It would be nice to have a versioning mechanism so that the version of the library is unambiguously correlated with the changes it contains.