Driver: Fix use of dangling std::string temporary

What's going on here is that the ternary operator produces a std::string rvalue
that the StringRef points to. I'd hoped bugs like this were a thing of the past
with our asan testing but apparently this code path is only used when LLVM is
configured with a custom --with-c-include-dirs setting.

Unbreaks bootstrapping with GCC5 on Fedora (PR22625), patch by Jonathan Wakely!

llvm-svn: 229719