Add sorry_at diagnostic function.

The plain "sorry" diagnostic only gives the "current" location, which is
typically the last line of the function or translation unit by time we get to
the back end.

GCN uses "sorry" to report unsupported language features, such as static
constructors, so it's useful to have a "sorry_at" variant.

This patch implements "sorry_at" according to the pattern of the other "at"
variants.

2018-09-11  Andrew Stubbs  <ams@codesourcery.com>

	gcc/
	* diagnostic-core.h (sorry_at): New prototype.
	* diagnostic.c (sorry_at): New function.

From-SVN: r264204