[clang] [Clang] Fix crash when ill-formed code is treated as a deduction guide (PR #67373)

[Shafik Yaghmour via cfe-commits]

shafik wrote:

I was originally going to roll-up this fix as part of: https://reviews.llvm.org/D148474 but it felt distinct enough to fix on its own and then rebase the other PR afterwards.

https://github.com/llvm/llvm-project/pull/67373