UniqueHeaderNameTest.cpp: Appease valgrind.

llvm::sys::path::native(dst, dst) might be overridden-safe in most cases, though.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@188800 91177308-0d34-0410-b5e6-96231b3b80d8