TableGen will produce 2 enums, RTLIB::Libcall and RTLIB::LibcallImpl. RTLIB::Libcall describes abstract functionality the compiler may choose to access, RTLIB::LibcallImpl describes a particular ABI implementation, which includes a name and type signature.
Declared in <llvm/IR/RuntimeLibcalls.h>
#define GET_RUNTIME_LIBCALL_ENUM