[libcxx] Add a missing include for __enable_if_t (#127016)
This is to fix compile error with explicit Clang modules like
```
../../third_party/libc++/src/include/__filesystem/path.h:80:26: error: declaration of '__enable_if_t' must be imported from module 'std_core.type_traits.enable_if' before it is required
80 | template <class _ECharT, __enable_if_t<__can_convert_char<_ECharT>::value, int> = 0>
| ^
../../third_party/libc++/src/include/__type_traits/enable_if.h:34:1: note: declaration here is not visible
34 | using __enable_if_t _LIBCPP_NODEBUG = typename enable_if<_Bp, _Tp>::type;
| ^
```
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