Minor C++ PATCHes to tsubsting

[Jason Merrill]

Two little things I noticed while working on another issue:

1) We were passing tf_none to tsubst_template_argument even when we want 
to explain a deduction failure.
2) We were calling tsubst_aggr_type and *then* checking whether it's a type.

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