Fix print of value type in a corner case of finish

Fix print of value type in a corner case of finish

When doing finish in a function, if gdb fails to return a value, gdb
also fails at printing the value type if this type is a struct.

For example :

(gdb) fin
....
Value returned has type: . Cannot determine contents

This patch fixes this by calling type_to_string to print the type
so that we can support these types.

This patch returns the following example output :

(gdb) fin
....
Value returned has type: struct test. Cannot determine contents

Also, this patch modifies structs.exp to check that we return the
correct type.

gdb/ChangeLog:
	* gdb/infcmd.c (print_return_value): use type_to_string to print type.

gdb/testsuite/ChangeLog:
	* gdb.base/structs.exp: Check for correct struct on finish.