[python] Fix gdb.Value.dynamic_type for reference values.

[python] Fix gdb.Value.dynamic_type for reference values.

gdb.Value.dynamic_type is supposed to work for reference and pointer
values.  However, the value object in the function 'valpy_get_dynamic_type'
was being dereferenced using 'value_ind' irrespective of the value type
being TYPE_CODE_PTR or TYPE_CODE_REF.  This patch fixes that to use
'coerce_ref' for TYPE_CODE_REF values.

ChangeLog:

	* python/py-value.c (valpy_get_dynamic_type): Use coerce_ref to
	dereference TYPE_CODE_REF values.

	testsuite/
	* gdb.python/py-value.c: Improve test case.
	* gdb.python/py-value.exp: Add new test.