Use IntrusiveRefCntPtr to manage the lifetime of BitCodeAbbrevs.

This doesn't change the interface or gives additional safety but removes
a ton of retain/release boilerplate.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217778 91177308-0d34-0410-b5e6-96231b3b80d8