[coroutine] Create coroutine body in the correct eval context (#78589)

Fixes: https://github.com/llvm/llvm-project/issues/78290



See the bug for more context.
```cpp
Gen ACoroutine() {
  if constexpr (0) // remove it make clang compile.
    co_return;
  co_await Gen{};
}
```
We miss symbol of ctor of promise_type if the first coroutine statement
happens to be inside the disabled branch of `if constexpr`.

This happens because the promise object is built when we see the first
coroutine statement which is present in
`ExpressionEvaluationContext::DiscardedStatement` context due to `if
constexpr (0)`. This makes clang believe that the promise constructor is
only odr-used and not really "used".

The expr evaluation context for the coroutine body should not be related
to the context in which the first coroutine statement appears. We
override the context to `PotentiallyEvaluated`.

---------

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