Add experimental support for SYCL under an experimental gate. The
IntelLLVM and Adaptivecpp compilers are recognized, as well as upstream
Clang with libsycl support.
Fixes: #27219, #27905
To manage the generation step, introduce cmGeneratorFileSet and
cmGeneratorFileSets classes.
These reorganizations of the code are done in preparation of the
implementation of SOURCES file set type as well as user's file set types.
Issues: #27550, #27383
Vito Gamberini