added test for undeclared functions

Follow up from #4211.  The type of function symbols must get fixed once the
function signature is known.

Author: Daniel Kroening

regression/ansi-c/undeclared_function/fileA.c

@@ -0,0 +1,7 @@
+// There is no #include <stdlib.h> here, deliberately,
+// to trigger automatic generation of a signature below.
+
+int main(void)
+{
+  malloc(4);
+}

regression/ansi-c/undeclared_function/fileB.c

@@ -0,0 +1,6 @@
+#include <stdlib.h>
+
+int f()
+{
+  malloc(4);
+}

regression/ansi-c/undeclared_function/undeclared_function1.desc

@@ -0,0 +1,8 @@
+KNOWNBUG
+fileA.c
+fileB.c --validate-goto-model
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+^CONVERSION ERROR$