[Support] [VirtualFileSystem] Detect the windows_slash path style

This fixes the following clang VFS tests, if `windows_slash` is the
default style:

  Clang :: VFS/implicit-include.c
  Clang :: VFS/relative-path.c
  Clang-Unit :: Frontend/./FrontendTests.exe/CompilerInstance.DefaultVFSOverlayFromInvocation

Also clarify a couple references to `Style::windows` into
`Style::windows_backslash`, to make it clearer that each of them are
opinionated in different directions (even if it doesn't matter for
calls to e.g. `is_absolute`).

Differential Revision: https://reviews.llvm.org/D113272