small C++ PATCH to instantiate_template_1

[Jason Merrill]

While looking at 55992 I noticed that we weren't pushing into dependent 
scope properly; we need to pass the "entering scope" flag to 
tsubst_aggr_type so that we get the version of the type with the 
members.  This isn't enough to fix 55992, but may well fix other bugs.

Tested x86_64-pc-linux-gnu, applying to trunk.
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