[clangd] Fixes in lit tests

Summary:
Changes:
- `background-index.test` Add Windows support.
- `did-change-configuration-params.test` Replace `cat | FileCheck` with `FileCheck --input-file`
- `test-uri-windows.test` This test did not run on Windows displite `REQUIRES: windows-gnu || windows-msvc` (replacement: `UNSUPPORTED: !(windows-gnu || windows-msvc)`).

Reviewers: sammccall, kadircet

Reviewed By: kadircet

Subscribers: njames93, ormris, ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83759