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

Fixes two issues in two ways:

1) The `braced-init-list` consisted of `initializer-list` and
`designated-initializer-list`, and thus the designated initializer is
subject to [over.match.class.deduct]p1.8, which means the brace elision
is also applicable on it for CTAD deduction guides.

2) When forming a deduction guide where the brace elision is applicable,
we should also consider the presence of braces within the initializer.
For example, given

template <class T, class U> struct X {
  T t[2];
  U u[3];
};

X x = {{1, 2}, 3, 4, 5};

we should establish such deduction guide AFAIU: `X(T (&&)[2], U, U, U) -> X<T, U>`.

Fixes https://github.com/llvm/llvm-project/issues/64625
Fixes https://github.com/llvm/llvm-project/issues/83368