message: Bump version to 1.16.0dev
diff: #define ISPC_VERSION_MAJOR 1
  #define ISPC_VERSION_MINOR 15
  -#define ISPC_VERSION "1.15.0"
  +#define ISPC_VERSION "1.16.0dev"
  #include <llvm/Config/llvm-config.h>
  #define ISPC_LLVM_VERSION (LLVM_VERSION_MAJOR * 10000 + LLVM_VERSION_MINOR * 100)
  