[Ada] Better handling of dynamic types in ada_value_primitive_packed_val

[Ada] Better handling of dynamic types in ada_value_primitive_packed_val

There is some partial handling for dynamic types in
ada_value_primitive_packed_val, but this support was added
in a fairly ad hoc way, and actually only covered the situation
where OBJ is not NULL and its contents had not been fetched yet.
In addition, even in the cases that it does cover, it doesn't make
much sense. In particular, it was adjusting BIT_SIZE and SRC_LEN,
which are properties of the data to be extracted _from_, based
on TYPE's length once resolved, which is a property of the data
we want to extract _to_.

This patch hopefully adjust this function to handle dynamic types
correctly, and in all cases. It does so by unpacking the data into
a temporary buffer in order to use that buffer to resolve the type.
And _then_ creates the resulting value from that resolved type.

gdb/ChangeLog:

        * ada-lang.c (ada_value_primitive_packed_val): Rework handling
        of case where TYPE is dynamic.