Consistently make llvm::opt::ArgStringList usage explicit to try and appease MSVC 2015 buildbots. NFCI.

llvm-svn: 344398
diff --git a/clang/lib/Driver/ToolChains/CrossWindows.cpp b/clang/lib/Driver/ToolChains/CrossWindows.cpp
index 6ca04a8..7953560 100644
--- a/clang/lib/Driver/ToolChains/CrossWindows.cpp
+++ b/clang/lib/Driver/ToolChains/CrossWindows.cpp
@@ -20,6 +20,7 @@
 using namespace clang::driver::toolchains;
 
 using llvm::opt::ArgList;
+using llvm::opt::ArgStringList;
 
 void tools::CrossWindows::Assembler::ConstructJob(
     Compilation &C, const JobAction &JA, const InputInfo &Output,