cgen: unwrap and c-mangle field selectors in or blocks

[edam]

This PR fixes the case where C keywords are not correctly mangled in field selectors with or blocks.

The following code otherwise generates invalid C:

struct Foo {
    long ?string // field is C keyword
}
fn main() {
    foo := Foo{''}
    println(foo.long or {''}) // generates invalid code
}

The following C code is generated:

VV_LOCAL_SYMBOL void main__main(void) {
	_option_string _t1;
	_option_ok(&(string[]) { _SLIT("") }, (_option*)(&_t1), sizeof(string));
	main__Foo f = ((main__Foo){._v_long = _t1,});
	_option_string _t2 = f.long;  // ERROR: field name is keyword
	if (_t2.state != 0) {
		IError err = _t2.err;
		*(string*) _t2.data = _SLIT("");
	}
	println( (*(string*)_t2.data));
}

There is no open ticket for this issue (that I'm aware of).

PR includes test.

[spytheman]

The CI failure is unrelated (it is a timed out slow running github job).

[spytheman]

Good work.