[RS4GC] Add a GCStrategy option to enable RS4GC

Currently, the RewriteStatepointsForGC pass has a hardcoded list of GC
strategies that it operates on, forcing downstream projects to patch
LLVM to enable it for their own strategies. The diff D141110 changes
that to each GCStrategy having a flag to enable or disable RS4GC.

This patch adds that flag. It currently doesn't do anything, but is
provided so downstream projects can enable it to smooth the transition.

Reviewed By: dantrushin

Differential Revision: https://reviews.llvm.org/D142169