C++ PATCH for c++/63139 (variadic alias templates)

[Jason Merrill]

This testcase was breaking because multiple substitutions of <Ts..., 
ContainerEndA> into T... would lose the original arguments when we pull 
the pattern out of the intermediate expansion.  So the added assert 
guards against that happening again, and we avoid the situation by 
simplifying the handling of that situation: if we are substituting into 
a trivial expansion like T... we can just return the argument pack.

Tested x86_64-pc-linux-gnu, applying to trunk and 4.9.
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