[Clang] [Sema] Don't crash on unexpanded pack in invalid block literal (#110762)

Consider #109148:
```c++
template <typename ...Ts>
void f() {
    [] {
        (^Ts);
    };
}
```

When we encounter `^Ts`, we try to parse a block and subsequently call
`DiagnoseUnexpandedParameterPack()` (in `ActOnBlockArguments()`), which
sees `Ts` and sets `ContainsUnexpandedParameterPack` to `true` in the
`LambdaScopeInfo` of the enclosing lambda. However, the entire block is
subsequently discarded entirely because it isn’t even syntactically
well-formed. As a result, `ContainsUnexpandedParameterPack` is `true`
despite the lambda’s body no longer containing any unexpanded packs,
which causes an assertion the next time
`DiagnoseUnexpandedParameterPack()` is called.

This pr moves handling of unexpanded parameter packs into
`CapturingScopeInfo` instead so that the same logic is used for both
blocks and lambdas. This fixes this issue since the
`ContainsUnexpandedParameterPack` flag is now part of the block (and
before that, its `CapturingScopeInfo`) and no longer affects the
surrounding lambda directly when the block is parsed. Moreover, this
change makes blocks actually usable with pack expansion.

This fixes #109148.

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