generator.yml: Add support for LLVM 15

* Add llvm_14 anchor and copy llvm_latest builds to llvm_14.
* Bump llvm_latest to 15 and copy llvm_tot builds to llvm_latest (no
  changes).
* Add schedules.

Closes: ClangBuiltLinux#387
Signed-off-by: Nathan Chancellor <nathan@kernel.org>