Correctly classify pack expansions as NON_CANONICAL_UNLESS_DEPENDENT

Test coverage for non-dependent pack expansions doesn't demonstrate a
failure prior to this patch (a follow-up commit improving debug info
will cover this commit specifically) but covers a related hole in our
test coverage.

Reviewed by Richard Smith & Eli Friedman.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186261 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed