[23/N] [libcxx] Have lexically_normal return the path with preferred separators

[mstorsjo]

Not pushing this one through CI right now, to save on CI time a bit, as it's rather trivial. On non-windows platforms, make_preferred() is a nop, so this should be safe for other platforms.

[mstorsjo]

Rebased

[mstorsjo]

Retrigger CI

libcxx/src/filesystem/operations.cpp

path path::lexically_normal() const {
   path Result;
   Result.__pn_.reserve(Parts.size() + NewPathSize + NeedTrailingSep);
   for (auto& PK : Parts)
     Result /= PK.first;
 
   if (NeedTrailingSep)
     Result /= PS("");
 
+  Result.make_preferred();
   return Result;
 }
 
 static int DetermineLexicalElementCount(PathParser PP) {
   int Count = 0;
   for (; PP; ++PP) {
     auto Elem = *PP;
     if (Elem == PS(".."))