[clangd] Extract FsPath from file:// uri

Patch contributed by stanionascu!

rfc8089#appendix-E.2 specifies that paths can begin with a drive letter e.g. as file:///c:/.
In this case just consuming front file:// is not enough and the 3rd slash must be consumed to produce a valid path on windows.

The patch introduce a generic way of converting an uri to a filesystem path and back.

Differential Revision: https://reviews.llvm.org/D31401

llvm-svn: 299758