selfhost: alias-aware istaggedtype for nested-union match

`type error = !(invalid | overflow)` miscompiled — istaggedtype
only matched N_TTAGGED directly, so an `e: error` param spilled
as 8B scalar and the match's slot+8 read trailed into saved BP.

Mirror isstrtype's alias+bang unwrap; add resolvetagged() for
is/as/match sites that need the inner N_TTAGGED. Frame scan
counts via slotsize so wwstage stays byte-identical to cstage.
Unblocks lib/strconv.strerror.

selfhost/cmd/wcc/cgendecl.ww

@@ -199,7 +199,7 @@ fn cgfnparams(c: *cgen, params: *node) void = {
 				p = p.next;
 				continue;
 			};
-			if (istaggedtype(p.lhs)) {
+			if (istaggedtype(c, p.lhs)) {
 				let slot: i32 = slotsize(c, p.lhs);
 				let nw: i32 = slot / 8;
 				if (idx + nw <= 6) {
@@ -317,7 +317,7 @@ fn cgfn(c: *cgen, fn_: *node) void = {
 	let frame: i32 = 0;
 	for (scanp != nil) {
 		if (scanp.kind == nkind.N_PARAM) {
-			if (istaggedtype(scanp.lhs)) { frame += 24; }
+			if (istaggedtype(c, scanp.lhs)) { frame += slotsize(c, scanp.lhs); }
 			else { if (isslicetype(c, scanp.lhs)) { frame += 24; }
 			else { if (isstrtype(c, scanp.lhs)) { frame += 16; }
 			else { frame += 8; }; }; };