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

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.

Differential Revision: https://reviews.llvm.org/D98406

GitOrigin-RevId: cb2648e6f0fbed25ffbb6cb5d9b76d46f39001aa
1 file changed