c-decl: fix "inform" grouping and conditionalization

c-decl: fix "inform" grouping and conditionalization

Whilst working on SARIF output I noticed some places where followup notes
weren't being properly associated with their errors/warnings in c-decl.cc.

Whilst fixing those I noticed some places where we "inform" after a
"warning" without checking that the warning was actually emitted.

Fixed the various issues seen in gcc/c/c-decl.cc thusly.

gcc/c/ChangeLog:
	* c-decl.cc (implicitly_declare): Add auto_diagnostic_group to
	group the warning with any note.
	(warn_about_goto): Likewise to group error or warning with note.
	Bail out if the warning wasn't emitted, to avoid emitting orphan
	notes.
	(lookup_label_for_goto): Add auto_diagnostic_group to
	group the error with the note.
	(check_earlier_gotos): Likewise.
	(c_check_switch_jump_warnings): Likewise for any error/warning.
	Conditionalize emission of the notes.
	(diagnose_uninitialized_cst_member): Likewise for warning,
	conditionalizing emission of the note.
	(grokdeclarator): Add auto_diagnostic_group to group the "array
	type has incomplete element type" error with any note.
	(parser_xref_tag): Add auto_diagnostic_group to group warnings
	with their notes.  Conditionalize emission of notes.
	(start_struct): Add auto_diagnostic_group to group the
	"redefinition of" errors with any note.
	(start_enum): Likewise for "redeclaration of %<enum %E%>" error.
	(check_for_loop_decls): Likewise for pre-C99 error.

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