Clang is designed as an API from its inception, allowing it to be reused by source analysis tools, refactoring, IDEs (etc) as well as for code generation. GCC is built as a monolithic static compiler, which makes it extremely difficult to use as an API and integrate into other tools. Further, its historic design and current policy makes it difficult to decouple the front-end from the rest of the compiler.
via https://clang.llvm.org/comparison.html, with more points for/against each option at that page.
1) clang and LLVM APIs are so unstable that tight integration effectively requires maintaining your own fork.
2) One of the original complaints about GCC was that Stallman stonewalled addition of a richer, stable plugin API. Fast forward to today, GCC plugins are now first-class and, AFAIU, the plugins API is more stable and dependable. OTOH, turns out people aren't using plugins as much as originally thought, though the Linux kernel infrastructure does.