Remove BitcodeReader::setBufferOwned.

We do have use cases for the bitcode reader owning the buffer or not, but we
always know which one we have when we construct it.

It might be possible to simplify this further, but this is a step in the
right direction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211205 91177308-0d34-0410-b5e6-96231b3b80d8