parser: fix C struct nested anon struct language

[kbkpbot]

Fix issue #25807
force anon struct/union in C.struct keep the same language of outside.

To fix issue #25788 in another PR due to bootstrap problem.

[spytheman]

Why is this different for a C one?

[kbkpbot]

The V compiler handles C structs differently than V structs.

// V auto str functions:
static string SYSTEM_INFO_2_str(SYSTEM_INFO_2* it) { return indent_SYSTEM_INFO_2_str(it, 0);}
static string C___VAnonStruct1_str(C___VAnonStruct1* it) { return indent_C___VAnonStruct1_str(it, 0);}
static string C___VAnonStruct2_str(C___VAnonStruct2* it) { return indent_C___VAnonStruct2_str(it, 0);}

// V auto str functions:
static string main__DeepStruct_str(main__DeepStruct it) { return indent_main__DeepStruct_str(it, 0);}
static string main___VAnonStruct1_str(main___VAnonStruct1 it) { return indent_main___VAnonStruct1_str(it, 0);}
static string main___VAnonStruct2_str(main___VAnonStruct2 it) { return indent_main___VAnonStruct2_str(it, 0);}

As shown above, when the compiler encounters a C struct, it accesses the value via a pointer. In contrast, for a V struct, it passes the struct by value (making a copy).

To ensure auto_str works correctly with a V anonymous struct, a corresponding C___VAnonStruct1 instance must be created for it.

[kbkpbot]

We can't direct access a C anon struct

[spytheman]

It may be better in the long term, to pass the V struct for the generated auto str methods by a reference too, to keep things consistent 🤔.

[spytheman]

Excellent work.
Thanks @kbkpbot .