Fix to PR12262 - assertion when substituting explicit template arguments
does not substitute a sizeof-pack expression.
The solution is proposed by Richard Smith.
Differential Revision: http://llvm-reviews.chandlerc.com/D869


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186306 91177308-0d34-0410-b5e6-96231b3b80d8