[libcxx] [test] Use a string_view of the native path type in the concat test

[mstorsjo]

Summary

This makes sure that no extra allocations happen on windows, fixing
earlier errors in the DisableAllocationGuard (in the second case that
is modified).

This is split out from D98398.

[• Quuxplusone]

LGTM, although this pre-existing "manually type-cast to a pointer type to trigger the appropriate operator X*()" technique is hecka confusing. I would be tempted to give class MultiStringView some additional member function templates for sanity's sake: TC.rhs.as_string_view<path::value_type>() and perhaps TC.rhs.as_cstr<path::value_type>().

[curdeius]

LGTM.
I like Arthur's idea of not using operatorX*()... just saying :).

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

for (auto const & TC : Cases) {
       path LHS((const char*)TC.lhs);
       path RHS((const char*)TC.rhs);
       path& Ref = (LHS += RHS);
       assert(LHS == (const char*)TC.expect);
       assert(&Ref == &LHS);
     }
     {
       path LHS((const char*)TC.lhs);
-      std::string_view RHS((const char*)TC.rhs);
+      std::basic_string_view<path::value_type> RHS((const path::value_type*)TC.rhs);
       path& Ref = (LHS += RHS);
       assert(LHS == (const char*)TC.expect);
       assert(&Ref == &LHS);
     }
     doConcatSourceTest<char>    (TC);
     doConcatSourceTest<wchar_t> (TC);
     doConcatSourceTest<char16_t>(TC);
     doConcatSourceTest<char32_t>(TC);
// Do path test
         LIBCPP_ONLY(DisableAllocationGuard g);
         path& Ref = (LHS += RHS);
         assert(&Ref == &LHS);
       }
       assert(LHS == E);
     }
     {
       path LHS((const char*)TC.lhs);
-      std::string_view RHS((const char*)TC.rhs);
+      std::basic_string_view<path::value_type> RHS((const path::value_type*)TC.rhs);
       const char* E = TC.expect;
       PathReserve(LHS, StrLen(E) + 5);
       {
         LIBCPP_ONLY(DisableAllocationGuard g);
         path& Ref = (LHS += RHS);
         assert(&Ref == &LHS);
       }
       assert(LHS == E);