When revisiting `const T& range = f(T());`, we do mark the variable for
`range` as constexpr-unknown, but not the temporary variable we create
for `T()`. Change that.
Then we also need to ignore constexpr-unknown pointers in
`CheckInvoke()`.
Fixes https://github.com/llvm/llvm-project/issues/187775
... if we don't have a block scope available. This can happen in
`EvalEmitter` scenarios and can cause local variable blocks to be
prematurely converted to dead blocks. Attach `ScopeKind::Block` variable
to the root scope instead.