Fix left shifts of negative values in MipsDisassembler.

This bug was reported by UBSan.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216920 91177308-0d34-0410-b5e6-96231b3b80d8
unique_ptrify FileOutputBuffer::FileOutputBuffer

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216921 91177308-0d34-0410-b5e6-96231b3b80d8