diagnostics: move client data hooks into namespace/dir "diagnostics"

diagnostics: move client data hooks into namespace/dir "diagnostics"

No functional change intended.

Note that the tree-*.cc implementations are *not* moved into namespace
diagnostics.

gcc/ChangeLog:
	* diagnostic-format-html.cc: Update #include for move of
	diagnostic-client-data-hooks.h to diagnostics/client-data-hooks.h.
	* diagnostic-format-sarif.cc: Likewise.  Update for move of
	diagnostic_client_plugin_info to diagnostics::client_plugin_info.
	* diagnostic-format-text.cc: Likewise.
	* diagnostic.cc: Likewise.
	* diagnostic.h: Likewise.
	* diagnostic-client-data-hooks.h: Move to...
	* diagnostics/client-data-hooks.h: ...here, updating header guard.
	(class diagnostic_client_data_hooks): Move to...
	(class diagnostics::client_data_hooks): ...here.  Remove redundant
	"diagnostics::" prefixes.
	(class diagnostic_client_plugin_info): Move to...
	(class diagnostics::client_plugin_info): ...here.
	* libgdiagnostics.cc: : Update #include for move of
	diagnostic-client-data-hooks.h to diagnostics/client-data-hooks.h.
	Update for move of diagnostic_client_data_hooks to
	diagnostics::client_data_hooks, and for move of
	diagnostic_client_version_info to diagnostics::client_version_info.
	* tree-diagnostic-client-data-hooks.cc: Likewise.
	* tree-diagnostic.cc: Likewise.

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