Add new keyword spellings with cilk_ instead of _Cilk_

[VoxSciurorum]

When -fopencilk is used cilk_spawn, cilk_sync, cilk_for, cilk_reducer, and cilk_scope become keywords. The spellings prefixed with _Cilk are unaffected. They remain keywords in all compilation modes. They will not generate parallel code if Tapir is not enabled.

[neboat]

These changes seem to be working well. I was able to remove #include <cilk/cilk.h> from several Cilk codes and observe that they still compile and run with these changes. In addition, without #include <cilk/cilk.h>, once I install my VSCode extension to recognize the syntax-highlight the Cilk keywords (https://github.com/neboat/vscode-opencilk), VSCode recognizes and colors the Cilk keywords just like other control-flow keywords, as in this example:

The cilk_reducer keyword isn't properly recognized, however, as in this example:

I think there are some places in clang that recognize the _Hyperobject keyword but not cilk_reducer. I suspect that adding cilk_reducer in those places should resolve the issue.

[VoxSciurorum]

The cilk_reducer problem should be fixed now.

I also switched the base branch to dev/19.x.