Add 'dg-note', 'dg-lto-note'

That's 'dg-message "note: [...]"' with a twist: inhibit default notes pruning,
such that "if 'dg-note' is used at least once in a testcase, [notes] are not
pruned and instead must *all* be handled explicitly".

The rationale is that either you're not interested in notes at all (default
behavior of pruning all notes), but often, when you're interested in one note,
you're in fact interested in all notes, and especially interested if
*additional* notes appear over time, as GCC evolves.

	gcc/testsuite/
	* lib/gcc-dg.exp: Implement 'dg-note'.
	* lib/prune.exp: Likewise.
	* gcc.dg/vect/nodump-vect-opt-info-2.c: Use 'dg-note', and
	'dg-prune-output "note: ".
	* gfortran.dg/goacc/routine-external-level-of-parallelism-2.f: Use
	'dg-note', match up additional notes, one class of them with
	XFAILed 'dg-bogus'.
	* lib/lto.exp: Implement 'dg-lto-note'.
	* g++.dg/lto/odr-1_0.C: Use 'dg-lto-note', match up additional
	notes.
	* g++.dg/lto/odr-1_1.C: Likewise.
	* g++.dg/lto/odr-2_1.C: Likewise.
	libstdc++-v3/
	* testsuite/lib/prune.exp: Add note about 'dg-note'.
	gcc/
	* doc/sourcebuild.texi: Document 'dg-note'.