Avoid diagnostic code crashing when external front-ends use "C" mode

At the time of writing both the rust and the Ada external front-ends
generate .json_symtab files that use the "C" mode.  If the ansi-c
front-end is not loaded then these will crash when reporting some linking
errors.

Author: martin

src/symtab2gb/Makefile

@@ -4,6 +4,7 @@ SRC = \
   # Empty last line
 
 OBJ += \
+  ../ansi-c/ansi-c$(LIBEXT) \
   ../util/util$(LIBEXT) \
   ../goto-programs/goto-programs$(LIBEXT) \
   ../big-int/big-int$(LIBEXT) \

src/symtab2gb/symtab2gb_parse_options.cpp

@@ -12,6 +12,8 @@ Author: Diffblue Ltd.
 #include <iostream>
 #include <string>
 
+#include <ansi-c/ansi_c_language.h>
+
 #include <goto-programs/goto_convert_functions.h>
 #include <goto-programs/goto_model.h>
 #include <goto-programs/link_goto_model.h>
@@ -113,6 +115,8 @@ int symtab2gb_parse_optionst::doit()
     gb_filename = cmdline.get_value(SYMTAB2GB_OUT_FILE_OPT);
   }
   register_language(new_json_symtab_language);
+  // Workaround to allow external front-ends to use "C" mode
+  register_language(new_ansi_c_language);
   config.set(cmdline);
   run_symtab2gb(symtab_filenames, gb_filename);
   return CPROVER_EXIT_SUCCESS;