[Driver] Add missing include of std::set.

4ddd4ed added a use of std::set without including it. With some
recent libc++, std::set isn't included transitively causing build
failures.

Add explicit include.

Author: fhahn

clang/lib/Driver/ToolChains/AIX.cpp

@@ -17,6 +17,8 @@
 #include "llvm/ProfileData/InstrProf.h"
 #include "llvm/Support/Path.h"
 
+#include <set>
+
 using AIX = clang::driver::toolchains::AIX;
 using namespace clang::driver;
 using namespace clang::driver::tools;