sourcebuild.texi (Test Directives): Document dg-shouldfail.

gcc:

	* doc/sourcebuild.texi (Test Directives): Document dg-shouldfail.

gcc/testsuite:

	* lib/gcc-dg.exp (tool_load): Wrapper to support shouldfail tests.
	(dg-test): Support shouldfail tests.
	* lib/target-supports-dg.exp (dg-shouldfail): New.
	* gcc.test-framework/test-framework.awk: Handle shouldfail tests.
	* gcc.test-framework/dg-do-run-sf-exp-F.c: New test.
	* gcc.test-framework/dg-do-run-sf-exp-P.c: New test.
	* gcc.test-framework/dg-do-run-sft-exp-F.c: New test.
	* gcc.test-framework/dg-do-run-sft-exp-P.c: New test.
	* gcc.test-framework/dg-dox-run-sf-exp-XF.c: New test.
	* gcc.test-framework/dg-dox-run-sf-exp-XP.c: New test.

From-SVN: r115424