Remove the `hasPrototype()` restriction so that old style K&R
declarations of main work too.
For example the following has 2 params but no prototype.
```
int main(argc, argv)
int argc;
char *argv[];
{
return 0;
}
```
Also, use `getNumParams()` over `param_size()` which seems to be a more
direct way to get at the same information.
Also, add missing tests for this mangling.
Differential Revision: https://reviews.llvm.org/D127888
Sam Clegg