Push immediate function context while transforming lambdas in templates. (#89702)

The following program is [accepted](https://godbolt.org/z/oEc34Trh4) by
Clang, EDG, and MSVC, but rejected by Clang:
```cpp
#include <vector>

consteval auto fn() { return std::vector {1,2,3}; }

template <typename T = int>
void fn2() {
    (void)[]() consteval {
      for (auto e : fn()) {}
    };
}

void caller() {
    fn2();
}
```

The stated diagnostic is:
```cpp
<source>:8:21: error: call to consteval function 'fn' is not a constant expression
    8 |       for (auto e : fn()) {}
```

The body of the lambda should be evaluated as within an immediate
function context when the lambda is marked as `consteval`.

Co-authored-by: cor3ntin <corentinjabot@gmail.com>

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