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

clang/lib/Driver/ToolChains.cpp

@@ -3154,7 +3154,8 @@ void Linux::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
     SmallVector<StringRef, 5> dirs;
     CIncludeDirs.split(dirs, ":");
     for (StringRef dir : dirs) {
-      StringRef Prefix = llvm::sys::path::is_absolute(dir) ? SysRoot : "";
+      StringRef Prefix =
+          llvm::sys::path::is_absolute(dir) ? StringRef(SysRoot) : "";
       addExternCSystemInclude(DriverArgs, CC1Args, Prefix + dir);
     }
     return;