[Flang][OpenMP] Skip component symbols when creating extended values (#79657)

The `map` clause in OpenMP allows structure components to be specified
(unlike other clauses). Structure components do get their own symbols,
but these are not meant to be instantiated.

When a component reference is passed as an argument to the omp.target
op, it gets a corresponding parameter in the target op's entry block.
The original symbols are then bound to the same kind of an extended
value as before, but the value is now based on the parameters.

To handle structure components more gracefully, put their symbols on the
list of mapped objects, but skip them when creating extended values.

Fixes https://github.com/llvm/llvm-project/issues/79478.