[PATCH] Introduce bitset metadata format and bitset lowering pass.

[Peter Collingbourne]

Unfortunately I've found that this won't work if a pass (such as globaldce) calls removeDeadConstantUsers on the vtable global, as this will cause the getelementptr to be deleted and replaced with null in the metadata.

I'll try to fix this by changing the metadata format to be of the form:

  { bitset name, global, byte offset }


http://reviews.llvm.org/D7288

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