Since you're talking about Clang, I assume you're coding in C? In that case, Coccinelle [https://coccinelle.gitlabpages.inria.fr/website/sp.html] may serve your purposes, and has a long history of use in the Linux kernel to modify shared APIs.
In case of Clang (and more specifically libClang,) I am usually using it to do source-to-source translations on C++ code. In those cases, sometimes I need some fairly advanced knowledge, such as the type of an expression, to match on or perform logic on. However, this still looks quite interesting for some use cases (and I am interested in such tools.)