C++ PATCH for c++/59012 (alignas pack expansion)

[Jason Merrill]

When we added support for C++11 attributes, we mentioned pack expansions 
in the comments but didn't do anything to support them in the code.  I'm 
not sure how that happened.  Anyway, this patch adds that support.

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