[C++ Patch/RFC] Tiny tsubst tweak

[Paolo Carlini <paolo dot carlini at oracle dot com>]

Hi,

while looking a bit into c++/71169 I noticed that in one place we first 
call tsubst_aggr_type and tubst_copy and then we check if either 
returned error_mark_node. Performance-wise, it would be better to check 
as soon as possible the return value of the former and in case return 
immediately without calling the latter. That assuming I'm not missing 
something of course... but the below passes testing.

Thanks,
Paolo.

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