Add a GCStrategy for CoreCLR

Add a GCStrategy for CoreCLR

This change adds a new GC strategy for supporting the CoreCLR runtime.

This strategy is currently identical to Statepoint-example GC,
but is necessary for several upcoming changes specific to CoreCLR, such as:

1. Base-pointers not explicitly reported for interior pointers
2. Different format for stack-map encoding
3. Location of Safe-point polls: polls are only needed before loop-back edges and before tail-calls (not needed at function-entry)
4. Runtime specific handshake between calls to managed/unmanaged functions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237753 91177308-0d34-0410-b5e6-96231b3b80d8