sizeof... a parameter pack should be an unevaluated context.

- can't think of a way to test this without generic lambda captures, but will include a test once that patch is made commit-ready.

patch was ok'd by Doug.
http://llvm-reviews.chandlerc.com/D2029 

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193757 91177308-0d34-0410-b5e6-96231b3b80d8