Make sure we don't lose default struct value when formatting struct

[compiler-errors]

The reason why rust-lang/rustfmt#6424 broke when #129514 landed is because the parser now successfully parses default struct values. That means that where we used to fail in rewrite_macro_inner:

rust/src/tools/rustfmt/src/macros.rs

Lines 263 to 267 in e108481

	let ParsedMacroArgs {
	args: arg_vec,
	vec_with_semi,
	trailing_comma,
	} = match parse_macro_args(context, ts, style, is_forced_bracket) {

... we now succeed, and we now proceed to format the inner struct as a macro arg. Since formatting was never implemented for default struct values, this means that we’ll always rewrite the struct to exclude the default value.

This PR makes it so that we simply don’t rewrite struct fields if they have a default value.

r? @ytmimi