cgen: fix eq for anon C structs

[dy-tea]

Given the below C struct (typedef is not required to repro):

// anon.h

typedef struct {
      struct {
               int x;
       } inner;
} outer;

// cstruct_anon_eq_test.c.v

#insert "@VMODROOT/anon.h"

@[typedef]
struct C.outer {
      inner struct {
            x int
      }
}

fn test_main() {
      a := C.outer{}
      b := C.outer{}
      assert a == b
}

Currently generates invalid eq functions:

======== Output of the C Compiler (/home/dylan/Downloads/v_linux/v/thirdparty/tcc/tcc.exe) ========
/tmp/v_1000/cstruct_anon_eq_test.01K37GBP8GYEP9GV6HW3RTH1D4.tmp.c:2416: error: cannot convert 'struct <anonymous>' to 'struct main___VAnonStruct1'
===================================================================================================
======== Output of the C Compiler (cc) ========
/tmp/v_1000/cstruct_anon_eq_test.01K37GBP8GYEP9GV6HW3RTH1D4.tmp.c: In function ‘indent_outer_str’:
/tmp/v_1000/cstruct_anon_eq_test.01K37GBP8GYEP9GV6HW3RTH1D4.tmp.c:2416:55: error: incompatible type for argument 1 of ‘indent_main___VAnonStruct1_str’
 2416 |         string _t1 = indent_main___VAnonStruct1_str(it->inner, indent_count + 1);
      |                                                     ~~^~~~~~~
      |                                                       |
      |                                                       struct <anonymous>
/tmp/v_1000/cstruct_anon_eq_test.01K37GBP8GYEP9GV6HW3RTH1D4.tmp.c:1886:66: note: expected ‘main___VAnonStruct1’ but argument is of type ‘struct <anonymous>’
 1886 | static string indent_main___VAnonStruct1_str(main___VAnonStruct1 it, int indent_count);
      |                                              ~~~~~~~~~~~~~~~~~~~~^~
/tmp/v_1000/cstruct_anon_eq_test.01K37GBP8GYEP9GV6HW3RTH1D4.tmp.c: In function ‘outer_struct_eq’:
/tmp/v_1000/cstruct_anon_eq_test.01K37GBP8GYEP9GV6HW3RTH1D4.tmp.c:2452:47: error: incompatible type for argument 1 of ‘main___VAnonStruct1_struct_eq’
 2452 |         return main___VAnonStruct1_struct_eq(a.inner, b.inner);
      |                                              ~^~~~~~
      |                                               |
      |                                               struct <anonymous>
/tmp/v_1000/cstruct_anon_eq_test.01K37GBP8GYEP9GV6HW3RTH1D4.tmp.c:2447:63: note: expected ‘main___VAnonStruct1’ but argument is of type ‘struct <anonymous>’
 2447 | inline bool main___VAnonStruct1_struct_eq(main___VAnonStruct1 a, main___VAnonStruct1 b) {
      |                                           ~~~~~~~~~~~~~~~~~~~~^
/tmp/v_1000/cstruct_anon_eq_test.01K37GBP8GYEP9GV6HW3RTH1D4.tmp.c:2452:56: error: incompatible type for argument 2 of ‘main___VAnonStruct1_struct_eq’
 2452 |         return main___VAnonStruct1_struct_eq(a.inner, b.inner);
      |                                                       ~^~~~~~
      |                                                        |
      |                                                        struct <anonymous>
/tmp/v_1000/cstruct_anon_eq_test.01K37GBP8GYEP9GV6HW3RTH1D4.tmp.c:2447:86: note: expected ‘main___VAnonStruct1’ but argument is of type ‘struct <anonymous>’
 2447 | inline bool main___VAnonStruct1_struct_eq(main___VAnonStruct1 a, main___VAnonStruct1 b) {
      |                                                                  ~~~~~~~~~~~~~~~~~~~~^
===============================================

struct <anonymous> and struct main___VAnonStruct1 are the same struct so we can cast to struct main___VAnonStruct1, which makes:

inline bool outer_struct_eq(outer a, outer b) {
	return main___VAnonStruct1_struct_eq(*(main___VAnonStruct1*)&(a.inner), *(main___VAnonStruct1*)&(b.inner));
}

I am unfamiliar with the codebase so let me know if something looks wrong.

[huly-for-github]

Connected to Huly®: V_0.6-23793

[spytheman]

Excellent work.
Thank you 🙇🏻‍♂️.