Add tests for Ada changes

Add tests for Ada changes

The previous patches largely came without test cases.  This was done
to make the patches easier to review; as most of the patches were
needed before existing tests could be updated.

This patch adds a new test and updates some existing tests to test all
the settings of -fgnat-encodings.  This ensures that tests are run
both with the old-style "magic symbol name" encoding, and the
new-style DWARF encoding.

Note that in one case, a test is modified to be more lax.  See the
comment in mi_var_array.exp.  I didn't want to fix this in this
series, as it's already complicated enough.  However, I think it could
be fixed; I will file a bug for it.

gdb/testsuite/ChangeLog
2020-04-24  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/mi_var_array.exp: Try all -fgnat-encodings settings.
	Make array type matching more lax.
	* gdb.ada/mi_var_union.exp: Try all -fgnat-encodings settings.
	* gdb.ada/mi_variant.exp: New file.
	* gdb.ada/mi_variant/pck.ads: New file.
	* gdb.ada/mi_variant/pkg.adb: New file.
	* gdb.ada/packed_tagged.exp: Try all -fgnat-encodings settings.
	* gdb.ada/unchecked_union.exp: Try all -fgnat-encodings settings.