Build two gdb.cp testcases with -Wno-unused-comparison

Build two gdb.cp testcases with -Wno-unused-comparison

Clang fails to compile two testcases with the following error:
  warning: equality comparison result unused [-Wunused-comparison]

This prevents the following testcases from executing:
  gdb.cp/koenig.exp
  gdb.cp/operator.exp

This commit builds those testcases with -Wno-unused-comparison, to
avoid the failure.  Note that this commit reveals a new failure,
"FAIL: gdb.cp/koenig.exp: p foo (p_union)" when the testsuite is
compiled using clang.

gdb/testsuite/ChangeLog:

	* gdb.cp/koenig.exp (prepare_for_testing): Add
	additional_flags=-Wno-unused-comparison.
	* gdb.cp/operator.exp (prepare_for_testing): Likewise.