[clang] [Clang][Modules] Fix crash when redefining default template argument (PR #185237)

[Younan Zhang via cfe-commits]

https://github.com/zyn0217 commented:

Note that if any of AI agent is involved, you're supposed to clarify it in your PR body. See our AI policy: https://llvm.org/docs/AIToolPolicy.html

Also this needs a release note.

https://github.com/llvm/llvm-project/pull/185237