Make compatible to CMake 4

Handle case in which -extra-args has more than one extra arg.

Update to the Clang v20.1.7 version of the main

Also make sure we build only the plugin, since the upstream
clang now supports loading clang-tidy plugins via the -load
option.

Use Clang provided public API

- Drop our own private copy of the headers
- Use the CMake Config

***

Support OMP

Merge pull request #42 from davidrohr/PR

Add -extra-args option

Bump to LLVM 18.1.8

Merge pull request #40 from davidrohr/PR

Bump to LLVM 17.0.6

Bump to Clang 13