Fix the build with libstdc++ v4.6 on Linux by supplying an explicit move
assignment operator. There is an issue where classes with deleted copy
constructors and user defined move constructors end up with an
implicitly deleted copy constructor that is selected instead of the
implicit move constructor. This is a bit of a hack, but it allows me to
build LLD in a bootstrap with libstdc++ which is my primary goal.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@172896 91177308-0d34-0410-b5e6-96231b3b80d8