[clang][C++20] P0960R3 and P1975R0: Allow initializing aggregates from a parenthesized list of values

This patch implements P0960R3, which allows initialization of aggregates
via parentheses.

As an example:

```
struct S { int i, j; };
S s1(1, 1);

int arr1[2](1, 2);
```

This patch also implements P1975R0, which fixes the wording of P0960R3
for single-argument parenthesized lists so that statements like the
following are allowed:

```
S s2(1);
S s3 = static_cast<S>(1);
S s4 = (S)1;

int (&&arr2)[] = static_cast<int[]>(1);
int (&&arr3)[2] = static_cast<int[2]>(1);
```

This patch was originally authored by @0x59616e and completed by
@ayzhao.

Fixes #54040, Fixes #54041

Co-authored-by: Sheng <ox59616e@gmail.com>

Full write up : https://discourse.llvm.org/t/c-20-rfc-suggestion-desired-regarding-the-implementation-of-p0960r3/63744

Reviewed By: ilya-biryukov

Differential Revision: https://reviews.llvm.org/D129531

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