[libcxx][test] Amend XFAIL directive in no_unique_address tests (#67849)

This PR amends the `XFAIL` directive to clang < 18 in
no_unique_address.compile.pass.cpp tests.

Since PR 67199 implements the [[msvc::no_unique_address]] attribute,
these tests pass in a windows-x64 build against ToT clang-cl:

```
********************
Unexpectedly Passed Tests (3):
llvm-libc++-static-clangcl.cfg.in :: libcxx/ranges/range.adaptors/range.lazy.split/no_unique_address.compile.pass.cpp
llvm-libc++-static-clangcl.cfg.in :: libcxx/ranges/range.adaptors/range.split/no_unique_address.compile.pass.cpp
llvm-libc++-static-clangcl.cfg.in :: libcxx/ranges/range.factories/range.istream.view/no_unique_address.compile.pass.cpp
```

---------

Co-authored-by: Caslyn Tonelli <caslyn@google.com>