stage2 was adding bogus error return trace frames when an error was not
being returned. This commit makes several improvements:

 * Make a runtime check if necessary to only emit a frame into the error
   return trace when an actual error is returned.
 * Use the `analyzeIsNonErrComptimeOnly` machinery to avoid runtime
   checks when it is compile-time-known that the value is an error, or a
   non-error.
 * Make std.builtin.returnError take a non-optional stack trace pointer.

closes #12174