[clang] [Clang] Fix handling of brace ellison when building deduction guides (PR #94889)

[Younan Zhang via cfe-commits]

zyn0217 wrote:

@hokein I realized the entire patch could be significantly simplified, in which we just don't need to perform the brace elision on a braced initializer. Can you take a second look at it?

https://github.com/llvm/llvm-project/pull/94889