[libcxx] Sanitize paths before creating symlinks on windows

The MS STL does even more cleanup (corresponding to lexically_normal
I think), but this seems to be the very minimum needed for making the
symlinks work when the target path contains non-native paths.

Differential Revision: https://reviews.llvm.org/D91145

GitOrigin-RevId: f65ba25cf37a57dc87db7af389c9dc637ca7dd8c
2 files changed