Correct unique_ptr passing in MCObjectDisassembler::setFallbackRegion

Rather than passing by lvalue reference, pass by value to ensure that
the caller provides an rvalue (and use move assignment, rather than
release+reset, to assign to the member variable)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216916 91177308-0d34-0410-b5e6-96231b3b80d8