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
List of changes:
1. Remove unneeded aliases
2. Don't write VS Solution on Apple
3. Use external script to perform clean
4. Fix small bug when empty environment was
resulting in invalid file.
It greatly reduces the size of .fdb file and
makes FASTBuild run much faster in different cases.
Fix `ApplyLinkRuleLauncher()` to apply `RULE_LAUNCH_COMPILE`.
Remove ApplyCompileRuleLauncher since it's not used and `RULE_LAUNCH_COMPILE` is handled in `AddCompilerLaunchersForLanguages()`
Fixes: #27183
Vito Gamberini