[clangd] Treat paths case-insensitively depending on the platform

Path{Match,Exclude} and MountPoint were checking paths case-sensitively
on all platforms, as with other features, this was causing problems on
windows. Since users can have capital drive letters on config files, but
editors might lower-case them.

This patch addresses that issue by:
- Creating regexes with case-insensitive matching on those platforms.
- Introducing a new pathIsAncestor helper, which performs checks in a
  case-correct manner where needed.

Differential Revision: https://reviews.llvm.org/D96690