[Support] Add a new path style for Windows with forward slashes

This behaves just like the regular Windows style, with both separator
forms accepted, but with get_separator() returning forward slashes.

Add a more descriptive name for the existing style, keeping the old
name around as an alias initially.

Add a new function `make_preferred()` (like the C++17
`std::filesystem::path` function with the same name), which converts
windows paths to the preferred separator form (while this one works on
any platform and takes a `path::Style` argument).

Contrary to `native()` (just like `make_preferred()` in `std::filesystem`),
this doesn't do anything at all on Posix, it doesn't try to reinterpret
backslashes into forward slashes there.

Differential Revision: https://reviews.llvm.org/D111879