Make the AttrListPtr object a part of the LLVMContext.

Make the AttrListPtr object a part of the LLVMContext.

When code deletes the context, the AttributeImpls that the AttrListPtr points to
are now invalid. Therefore, instead of keeping a separate managed static for the
AttrListPtrs that's reference counted, move it into the LLVMContext and delete
it when deleting the AttributeImpls.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@168354 91177308-0d34-0410-b5e6-96231b3b80d8