Revert "Make BitCodeAbbrev ownership explicit using shared_ptr rather than IntrusiveRefCntPtr"

Breaks Clang's use of bitcode. Reverting until I have a fix to go with
it there.

This reverts commit r291006.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291007 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed