[llvm-exegesis] Add missing std::move in rL374146.

This was breaking some bots:

/home/buildbots/ppc64le-clang-lnt-test/clang-ppc64le-lnt/llvm/include/llvm/Support/Error.h:483:5:   required from ‘llvm::Expected<T>::Expected(OtherT&&, typename std::enable_if<std::is_convertible<_Rep2, _Rep>::value>::type*) [with OtherT = std::vector<llvm::exegesis::CodeTemplate>&; T = std::vector<llvm::exegesis::CodeTemplate>; typename std::enable_if<std::is_convertible<_Rep2, _Rep>::value>::type = void]’
/home/buildbots/ppc64le-clang-lnt-test/clang-ppc64le-lnt/llvm/tools/llvm-exegesis/lib/X86/Target.cpp:238:20:   required from here
/usr/include/c++/6/bits/stl_construct.h:75:7: error: use of deleted function ‘llvm::exegesis::CodeTemplate::CodeTemplate(const llvm::exegesis::CodeTemplate&)’
     { ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

llvm-svn: 374149