Currently C API contains several deprecated functions,
however the deprecation mentioned only inside a comment around a function.
The patch replaces comment within attribute((deprecated("Use <alternative>"))).
Do we have to leave this in include/llvm-c? Could it be somewhere in include/llvm? We may already have a place for such macros.
This is exactly my intention. I copied the definition from llvm/Support/Compiler.h, but besides that it contains a lot of other stuff, that might not be suitable for C API (e.g. sanitizers).
Another possible solution is to introduce another header and include it into both llvm/Support/Compiler.h and C API interface.
Also sometimes we keep alive some C++ APIs just for the purpose of supporting a C API, I think we should mark these as deprecated as well so we know we can remove them.
This is a very good idea, I will skim through the implementations and check whether we can deprecate something there.