checker: fix treating C structs with capitalized fields as embeds (#8343)

vlib/v/checker/checker.v

@@ -2081,7 +2081,8 @@ pub fn (mut c Checker) selector_expr(mut selector_expr ast.SelectorExpr) table.T
 	mut unknown_field_msg := 'type `$sym.name` has no field or method `$field_name`'
 	mut has_field := false
 	mut field := table.Field{}
-	if field_name.len > 0 && field_name[0].is_capital() && sym.info is table.Struct {
+	if field_name.len > 0 && field_name[0].is_capital() && sym.info is table.Struct
+		&& sym.language == .v {
 		// x.Foo.y => access the embedded struct
 		sym_info := sym.info as table.Struct
 		for embed in sym_info.embeds {

vlib/v/tests/project_with_c_code/main.v

@@ -5,4 +5,5 @@ import mod1
 fn main() {
 	res := mod1.vadd(1, 2)
 	println(res)
+	assert res == 1003
 }

vlib/v/tests/project_with_c_code/mod1/c/header.h

@@ -3,4 +3,8 @@
 
 int cadd(int a, int b);
 
+struct MyStruct {
+    int UppercaseField;
+};
+
 #endif

vlib/v/tests/project_with_c_code/mod1/wrapper.v

@@ -5,8 +5,13 @@ module mod1
 
 #include "header.h"
 
-fn C.cadd(int,int) int
+struct C.MyStruct {
+	UppercaseField int
+}
+
+fn C.cadd(int, int) int
 
 pub fn vadd(a int, b int) int {
-	return 1000 + C.cadd(a,b)
+	x := C.MyStruct{ 100 }
+	return 900 + x.UppercaseField + C.cadd(a, b)
 }