[libcxx] [test] Use the native path types in path.compare

[mstorsjo]

Summary

This makes sure that it actually tests the right compare() overloads in windows configurations.

This also fixes the allocation guards that enforce no allocations while running the compare() functions.

[curdeius]

Great! LGTM. Thanks for fixing that.

libcxx/test/std/input.output/filesystems/class.path/path.member/path.compare.pass.cpp

 }
 
 void test_compare_basic()
 {
   using namespace fs;
   for (auto const & TC : CompareTestCases) {
     const path p1(TC.LHS);
     const path p2(TC.RHS);
-    const std::string R(TC.RHS);
-    const std::string_view RV(TC.RHS);
+    std::string RHS(TC.RHS);
+    const path::string_type R(RHS.begin(), RHS.end());
+    const std::basic_string_view<path::value_type> RV(R);
+    const path::value_type *Ptr = R.c_str();
     const int E = TC.expect;
     { // compare(...) functions
       DisableAllocationGuard g; // none of these operations should allocate
 
       // check runtime results
       int ret1 = normalize_ret(p1.compare(p2));
       int ret2 = normalize_ret(p1.compare(R));
-      int ret3 = normalize_ret(p1.compare(TC.RHS));
+      int ret3 = normalize_ret(p1.compare(Ptr));
       int ret4 = normalize_ret(p1.compare(RV));
 
       g.release();
       assert(ret1 == ret2);
       assert(ret1 == ret3);
       assert(ret1 == ret4);
       assert(ret1 == E);