Worse diagnostic for init-captured lambda appearing within a brace initializer

[zyn0217]

struct S {
  template <class T>
  S(T) {}
};

void foo() {
  S s([a(42), &] {}); // Clang diagnoses 'capture default must be first' which is very clear
}

However, if we initialize S with a brace initializer,

struct S {
  template <class T>
  S(T) {}
};

void foo() {
  S s{[a(42), &] {}};
}

Clang diagnoses

<source>:7:8: error: use of undeclared identifier 'a'
    7 |   S s{[a(42), &] {}};
      |        ^
<source>:7:13: error: expected ']'
    7 |   S s{[a(42), &] {}};
      |             ^
<source>:7:7: note: to match this '['
    7 |   S s{[a(42), &] {}};
      |       ^
<source>:7:7: warning: array designators are a C99 extension [-Wc99-designator]
    7 |   S s{[a(42), &] {}};
      |       ^~~~~~~~~~

for which clang does seem to parse it as an array designator and incurs confusing diagnostics.

See it on godbolt:

https://godbolt.org/z/xq6GfEbcx

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Younan Zhang (zyn0217)

```cpp struct S { template <class T> S(T) {} };

void foo() {
S s([a(42), &] {}); // Clang diagnoses 'capture default must be first' which is very clear
}

However, if we initialize S with a brace initializer,

```cpp
struct S {
  template <class T>
  S(T) {}
};

void foo() {
  S s{[a(42), &] {}};
}

Clang diagnoses

<source>:7:8: error: use of undeclared identifier 'a'
    7 |   S s{[a(42), &] {}};
      |        ^
<source>:7:13: error: expected ']'
    7 |   S s{[a(42), &] {}};
      |             ^
<source>:7:7: note: to match this '['
    7 |   S s{[a(42), &] {}};
      |       ^
<source>:7:7: warning: array designators are a C99 extension [-Wc99-designator]
    7 |   S s{[a(42), &] {}};
      |       ^~~~~~~~~~

for which clang does seem to parse it as an array designator and incurs confusing diagnostics.

See it on godbolt:

https://godbolt.org/z/xq6GfEbcx