[clang-tools-extra] fix parse windows driver and provides support for WSL paths. (PR #119350)

[via cfe-commits]

95833 wrote:

/var/lib/buildkite-agent/builds/linux-56-59b8f5d88-fkcjf-1/llvm-project/github-pull-requests/clang-tools-extra/clangd/unittests/PathMappingTests.cpp:34
Value of: failedParse(R"(C:a=/root)")
  Actual: false
Expected: true

https://github.com/llvm/llvm-project/pull/119350