Fixes #163498

---

This PR addresses the issue of confusing diagnostics for lambdas with
init-captures appearing inside braced initializers.

Cases such as:

```cpp
S s{[a(42), &] {}};
```

were misparsed as C99 array designators, producing unrelated
diagnostics, such as `use of undeclared identifier 'a'`, and `expected
']'`

---


https://github.com/llvm/llvm-project/blob/bb9bd5f263226840194b28457ddf9861986db51f/clang/lib/Parse/ParseInit.cpp#L470


https://github.com/llvm/llvm-project/blob/bb9bd5f263226840194b28457ddf9861986db51f/clang/lib/Parse/ParseInit.cpp#L74


https://github.com/llvm/llvm-project/blob/bb9bd5f263226840194b28457ddf9861986db51f/clang/include/clang/Parse/Parser.h#L4652-L4655


https://github.com/llvm/llvm-project/blob/24c22b7de620669aed9da28de323309c44a58244/clang/lib/Parse/ParseExprCXX.cpp#L871-L879

The tentative parser now returns `Incomplete` for partially valid lambda
introducers, preserving the `lambda` interpretation and allowing the
proper diagnostic to be issued later.

---

Clang now correctly recognizes such constructs as malformed lambda
introducers and emits the expected diagnostic — for example,
“capture-default must be first” — consistent with direct initialization
cases such as:

```cpp
S s([a(42), &] {});
```

Currently a capture-default which is not the first element in the lambda-capture
is diagnosed with a generic expected variable name or 'this' in lambda capture
list, which is true but not very helpful.

If we don't have already parsed a capture-default then a lone "&" or "=" is
likely to be a misplaced capture-default, so diagnose it as such.

Differential Revision: https://reviews.llvm.org/D83681

Reviewed By: aaron.ballman