re PR preprocessor/60570 (expression in 'elif' directive mis-diagnosed as error when group will be skipped)

	DR#412
	PR preprocessor/60570
	* directives.c (do_elif): Don't evaluate #elif conditionals
	when they don't need to be.

	* gcc.dg/cpp/pr36320.c: Turn dg-error into dg-bogus.
	* gcc.dg/cpp/pr60570.c: New test.

From-SVN: r220035