[Clang][Driver] Fix include paths for `--sysroot /` on Linux

Currently if `--sysroot /` is passed to the Clang driver, the include paths generated by the Clang driver will start with a double slash: `//usr/include/...`.
 If VFS is used to inject files into the include paths (for example, the Swift compiler does this), VFS will get confused and the injected files won't be visible.

This change makes sure that the include paths start with a single slash.

Fixes #28283.

Differential Revision: https://reviews.llvm.org/D126289