Fix undefined behavior.
Fix undefined behavior.

Having std:move(mb) and mb->getBuffer() in the same argument list is not safe
because the order of evaluation is not defined.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@234541 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld