3C crash when same function has declaration and definition in different files with different parameter types

[mattmccutchen-cci]

3C crashes when the same function has a declaration and a definition in different files with different parameter types. This is the same original problem as #283, but with a declaration and a definition instead of two declarations. An example is 3c -output-postfix=checked mismatch1.c mismatch2.c with:

mismatch1.c:

void foo(char *x);

mismatch2.c:

void foo(char **y) {
  // this is the body
}

I hope this can be fixed by simply applying the same safety check we currently have in the two-declaration case (from #330) to the "declaration and definition" case.