C++ PATCH for c++/66289 (ICE with alias-template)

[Jason Merrill]

My patch for DR 1558 broke this testcase: suppressing the stripping of 
alias template-ids with dependent arguments prevented us from 
recognizing that APtr<T> is more specialized than shared_ptr<T>.  At the 
Lenexa meeting we in Core talked about establishing a category of alias 
templates that are actually equivalent to their expansions, based 
largely on whether all the template parameters are used.  This patch 
implements that idea, and fixes the testcase.

Tested x86_64-pc-linux-gnu, applying to trunk and 5.
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