Introduce lazily-created.h

Introduce lazily-created.h

No functional change intended.

gcc/ChangeLog:
	* diagnostics/context.cc: Eliminate digraphs::lazy_digraph in
	favor of lazily_created template.
	* diagnostics/context.h: Likewise.
	* diagnostics/digraphs.cc: Likewise, also digraphs::lazy_digraphs.
	* diagnostics/digraphs.h: Likewise.
	* diagnostics/html-sink.cc: Likewise.
	* diagnostics/metadata.h: Likewise.
	* diagnostics/sarif-sink.cc: Likewise.
	* diagnostics/sink.h: Likewise.
	* diagnostics/text-sink.h: Likewise.
	* lazily-created.h: New file.
	* libgdiagnostics.cc: Eliminate digraphs::lazy_digraph in
	favor of lazily_created template.

gcc/testsuite/ChangeLog:
	* gcc.dg/plugin/diagnostic_plugin_test_graphs.cc: Eliminate
	digraphs::lazy_digraph and digraphs::lazy_digraphs in favor of
	lazily_created template.
	* gcc.dg/plugin/diagnostic_plugin_test_metadata.cc: Define
	INCLUDE_VECTOR since diagnostics/metadata.h now requires it.
	* gcc.dg/plugin/diagnostic_plugin_test_paths.cc: Likewise.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>