Rollup merge of #161661 - maxdexh:least-cursed-windows-feature, r=ChrisDenton

Don't treat slashes as path seps after drive letters in verbatim paths

Treat slashes as just a regular character in `\\?\C:/path\somewhere`. This mirrors how slashes are treated afterwards, e.g. in `\\?\C:\path/somewhere`, which has a component `path/somewhere`, rather than `path`, `somewhere`.

Likewise, drive letters followed by `/` do not denote a `VerbatimDisk`, but an arbitrary `Verbatim` path: The prefix of `\\?\C:/path\somewhere` is `\\?\C:/path`.

Fixes rust-lang/rust#161651