[Sema] Don't diagnose an array type mismatch when the new or previous

declaration has a dependent type.

This fixes a bug where clang errors out on a valid code.

rdar://problem/28051467

Differential Revision: https://reviews.llvm.org/D24110


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@280330 91177308-0d34-0410-b5e6-96231b3b80d8