[clangd] Handle lambda scopes inside Node::getDeclContext() (#76329)

We used to consider the `DeclContext` for selection nodes inside a
lambda as the enclosing scope of the lambda expression, rather than the
lambda itself.

For example,

```cpp
void foo();
auto lambda = [] {
  return ^foo();
};
```

where `N` is the selection node for the expression `foo()`,
`N.getDeclContext()` returns the `TranslationUnitDecl` previously, which
IMO is wrong, since the method `operator()` of the lambda is closer.

Incidentally, this fixes a glitch in add-using-declaration tweaks.
(Thanks @HighCommander4 for the test case.)

GitOrigin-RevId: 9ef2ac3ad1bd5aa9e589f63047e8abeac11ad1b2