Use getLastArgValue instead of getString

Summary: I found that getString defined in the LLD's Driver.cpp is
exactly the same as Arg::getLastArgValue defined in
llvm/Option/ArgLIst.h. This patch removes that local function and use
the function that the Arg class provides.

Reviewers: ruiu

Subscribers: emaste, llvm-commits

Differential Revision: https://reviews.llvm.org/D34196

llvm-svn: 305374