[Sema] Fix a crash in access checking for deduction guides

Summary: See the added test for a repro.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58111

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@353840 91177308-0d34-0410-b5e6-96231b3b80d8