[RS4GC] Remove the hardcoded GC strategy names (v2)

Previously, RewriteStatepointsForGC had a hardcoded list of GC
strategies for which it would run, and using it with a custom strategy
required patching LLVM.

The logic for selecting the variables that are considered managed was
also hardcoded to use pointers in address space 1, rather than
delegating to GCStrategy::isGCManagedPointer.

This patch fixes both of these flaws: this pass now applies to all
functions whose GCStrategy returns true for useStatepoints, and checking
if a pointer is managed or not is also now done by the strategy.

One potentially questionable design decision in this change: the pass will
be enabled for all GC strategies that use statepoints. It seems unlikely
this would be a problem - consumers that don't use this pass probably
aren't adding it to the pass manager anyway - but if you had two different
GC strategies and only one wants this pass enabled then that'd need a new
flag in GCStrategy, which I can add if anyone thinks it's necessary.

This is an updated version of D140458, rebased to account for LLVM's
changes since D140504 (required by this patch) landed.

Reviewed By: dantrushin

Differential Revision: https://reviews.llvm.org/D141110

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