[clang] Deduplicate FinishTemplateArgumentDeduction helper function in SemaTemplateDeduction.cpp.

[hokein]

In SemaTemplateDeduction.cpp, we have two duplicated versions of FinishTemplateArgumentDeduction. And after #89358, we have three duplications.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Haojian Wu (hokein)

In `SemaTemplateDeduction.cpp`, we have two duplicated versions of [`FinishTemplateArgumentDeduction`](https://github.com/llvm/llvm-project/blob/main/clang/lib/Sema/SemaTemplateDeduction.cpp#L3192). And after #89358, we have three duplications.

[llvmbot]

Hi!

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[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Haojian Wu (hokein)

In `SemaTemplateDeduction.cpp`, we have two duplicated versions of [`FinishTemplateArgumentDeduction`](https://github.com/llvm/llvm-project/blob/main/clang/lib/Sema/SemaTemplateDeduction.cpp#L3192). And after #89358, we have three duplications.

[Rajveer100]

@hokein
The refactoring would decrease the overall lines of code, but don't you think it would be a little clumsy to make it a single function, let me know what's your approach?