[PlaceSafepoints] Adjust enablement logic to default to off and be GC configurable per GC

Previously, this pass ran over every function in the Module if added to the pass order.  With this change, it runs only over those with a GC attribute where the GC explicitly opts in.  A GC can also choose which of entry safepoint polls, backedge safepoint polls, and call safepoints it wants.  I hope to get these exposed as checks on the GCStrategy at some point, but for now, the checks are manual string comparisons.

llvm-svn: 230097