fix: error on non-exhaustive enums with zero width backing type (#21374)

Co-authored-by: WillLillis <wlillis@umass.edu>

test/cases/compile_errors/zero_width_nonexhaustive_enum.zig

@@ -0,0 +1,17 @@
+comptime {
+    _ = enum(i0) { a, _ };
+}
+
+comptime {
+    _ = enum(u0) { a, _ };
+}
+
+comptime {
+    _ = enum(u0) { a, b, _ };
+}
+
+// error
+//
+// :2:9: error: non-exhaustive enum specifies every value
+// :6:9: error: non-exhaustive enum specifies every value
+// :10:23: error: enumeration value '1' too large for type 'u0'