Fix byval args (e.g. large structs)

[hikari-no-yume]

This must have regressed at some point, because the existence of ByValParams suggests there used to be handling of it.

The problem is that if you have a byval struct argument (produced for e.g. void foo(some_large_struct bar)) then it's a pointer type in LLVM, but in C it's a by-value struct argument, so we need to add & in front of it.

[hikari-no-yume]

Example of the problem:

$ cc ../testfiles/byval-simpler.cbe.c 
../testfiles/byval-simpler.cbe.c:49:14: error: member reference type 'struct
      l_struct_struct_OC_intbox' is not a pointer; did you mean to use '.'?
  _4 = *((&_1->field0));
           ~~^~
             .
../testfiles/byval-simpler.cbe.c:49:11: error: cannot take the address of an
      rvalue of type 'uint32_t' (aka 'unsigned int')
  _4 = *((&_1->field0));
          ^~~~~~~~~~~
../testfiles/byval-simpler.cbe.c:50:14: error: member reference type 'struct
      l_struct_struct_OC_intbox' is not a pointer; did you mean to use '.'?
  _5 = *((&_2->field0));
           ~~^~
             .
../testfiles/byval-simpler.cbe.c:50:11: error: cannot take the address of an
      rvalue of type 'uint32_t' (aka 'unsigned int')
  _5 = *((&_2->field0));
          ^~~~~~~~~~~
4 errors generated.

(This is the test included in the patch.)