C++ PATCH for c++/68422 (sizeof... slow to evaluate)

[Jason Merrill]

This turns out to be because we represent sizeof... as a sizeof a pack 
expansion internally, and we generated a full new pack expansion one 
element at a time rather than just look at how many elements there were 
in the argument pack.

We already had an optimization in tsubst_parameter_pack to recognize 
when the pack expansion we're asking for will be the same as one of the 
argument packs, but we weren't doing it for expressions because we might 
need to convert_from_reference the elements.  This patch improves the 
optimization to recognize when we won't need to worry about that, and 
therefore handle more expression pack expansions, which fixes the 
quadratic behavior on the testcase.

But we'd like sizeof... to be fast for packs of references as well, so I 
added a flag to EXPR_PACK_EXPANSION for sizeof... so we can always just 
return the argument pack.  Surprisingly, this provided a small but 
consistent further speedup for the testcase.

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