Driver: Factor string vector push_back into a helper class and optimize it.

Due to lack of move semantics we would create a temporary std::string from the
string literal, copy it into the vector and discard the temporary. This leads
to massive code bloat, optimizing it saves 50k on i386-linux-Release+Asserts.

While there add a two-element overload for push_back, simplifying code a bit.

llvm-svn: 153441