Enabling the target-independent garbage collection infrastructure by hooking it

Enabling the target-independent garbage collection infrastructure by hooking it
up to the various compiler pipelines.

This doesn't actually add support for any GC algorithms, which means it
temporarily breaks a few tests. To be fixed shortly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@45669 91177308-0d34-0410-b5e6-96231b3b80d8