Add dg-line

Add dg-line

Context: there are currently two types of line number supported in
dg-{error,warning,message,bogus} directives: absolute and relative.  With an
absolute line number, it's immediately clear what line number is meant, but
when a line is added at the start of the file, the line number needs to be
updated.  With a relative line number, that problem is solved, but when relative
line numbers become large, it becomes less clear what line it refers to, and
when adding a line inbetween the directive using the relative line number and
the line it refers to, the relative line number still needs to be updated.

Add a directive dg-line with argument varname, that saves the line number
of the directive in a variable varname, which can be used as line number in dg
directives.

2017-04-25  Tom de Vries  <tom@codesourcery.com>

	* lib/gcc-dg.exp (cleanup-after-saved-dg-test): Cleanup line number
	variables.
	(dg-line): New proc.
	(process-message): Handle line number variables.
	* objc.dg/try-catch-12.m: Use dg-line.

From-SVN: r247251