bitfield-parent-optimized-out: Fix struct definition

The "struct S" type in bitfield-parent-optimized-out.exp is declared to
have a size of 4 bytes but to hold two 4-byte members: an int-based
bitfield and a 4-byte int.  Also, both members have the same
data_member_location 2, causing them to overlap and to reach 2 bytes
beyond the structure's boundary.

This is fixed by increasing the structure size to 8 and setting the
first and second member's data_member_location to 0 and 4, respectively.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/bitfield-parent-optimized-out.exp: Fix DWARF code for
	the definition of struct S.