Auto merge of #15179 - ponyii:fix/default-values-of-const-params-are-ignored, r=HKalbasi

the "add missing members" assists: implemented substitution of default values of const params

To achieve this, I've made `hir::ConstParamData` store the default values

crates/syntax/src/ast/make.rs

@@ -503,11 +503,16 @@ pub fn hacky_block_expr(
 pub fn expr_unit() -> ast::Expr {
     expr_from_text("()")
 }
+
 pub fn expr_literal(text: &str) -> ast::Literal {
     assert_eq!(text.trim(), text);
     ast_from_text(&format!("fn f() {{ let _ = {text}; }}"))
 }
 
+pub fn expr_const_value(text: &str) -> ast::ConstArg {
+    ast_from_text(&format!("trait Foo<const N: usize = {text}> {{}}"))
+}
+
 pub fn expr_empty_block() -> ast::Expr {
     expr_from_text("{}")
 }