[clangd] Fix conversion from Windows UNC paths to file URI format.

Summary:
The fix improves handling of Windows UNC paths to align with Appendix E. Nonstandard Syntax Variations of RFC 8089.

Before this fix it was difficult to use Windows UNC paths in compile_commands.json database as such paths were converted to file URIs using 'file:////auth/share/file.cpp' notation instead of recommended 'file://auth/share/file.cpp'.

As an example, VS.Code cannot understand file URIs with 4 starting slashes, thus such features as go-to-definition, jump-to-file, hover tooltip, etc. stop working. This also applicable to files which reside on Windows network-mapped drives because clangd internally resolves file paths to real paths in some cases and such paths get resolved to UNC paths.

Reviewers: sammccall, kadircet

Reviewed By: sammccall

Subscribers: ormris, ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, kbobyrev, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D84172