Document special use of target_type field in Ada structs.

Document special use of target_type field in Ada structs.

This is a comment that I wrote not too long ago, I believe, as part
of a submission process, and somehow did not get to commit.  It explains
how the TYPE_TARGET_TYPE is used by Ada in the case of dynamic records/
unions.

2010-04-24  Joel Brobecker  <brobecker@adacore.com>

        * gdbtypes.h (struct main_type): Expand comment about target_type
        field.