added _Noreturn for clang

git-svn-id: svn+ssh://svn.cprover.org/srv/svn/cbmc/trunk@6023 6afb6bc1-c8e4-404c-8f48-9ae832c5b171

Author: kroening

src/ansi-c/c_preprocess.cpp

@@ -707,6 +707,9 @@ bool c_preprocess_gcc_clang(
   command+=" -D__DEC128_MANT_DIG__=34";
   command+=" -D__LDBL_MIN_10_EXP__=\"(-4931)\"";
 
+  if(preprocessor==configt::ansi_ct::preprocessort::PP_CLANG)
+    command+=" -D_Noreturn=__attribute__((__noreturn__))";
+
   if(config.ansi_c.int_width==16)
     command+=GCC_DEFINES_16;
   else if(config.ansi_c.int_width==32)