Check if clazy should be linked to clang-cpp

Summary:
According to [1], clang can now provide a single shared library instead of split
ones.

We have to check if this library exists and link to it if available.

[1] https://releases.llvm.org/9.0.0/tools/clang/docs/ReleaseNotes.html#build-system-changes

Reviewers: smartins, kde-buildsystem

Reviewed By: smartins

Differential Revision: https://phabricator.kde.org/D25163