[Sema] Fix the assertion in Sema::ActOnDependentMemberExpr

61700724 introduced the use of ActOnDependentMemberExpr with
variable template specialization. The assertion inside
ActOnDependentMemberExpr should be adjusted accordingly.

Fixes https://bugs.llvm.org/show_bug.cgi?id=47211

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D113146