It seems that the current ClangFormat plugin can't be built with a system clang installation. It uses the define  KEEP_LINE_BREAKS_FOR_NON_EMPTY_LINES_BACKPORTED for something that seems not even yet accepted in clang's upstream git master https://reviews.llvm.org/D53072

Even more, I can't see where the definition will come from from the embedded clang/llvm code either, so maybe this is broken even for full embedded sources too?