Be pickier about converting between Name and Datum.

We were misapplying NameGetDatum() to plain C strings in some places.
This worked, because it was just a pointer cast anyway, but it's a type
cheat in some sense.  Use CStringGetDatum instead, and modify the
NameGetDatum macro so it won't compile if applied to something that's
not a pointer to NameData.  This should result in no changes to
generated code, but it is logically cleaner.

Mark Dilger, tweaked a bit by me

Discussion: <EFD8AC94-4C1F-40C1-A5EA-304080089C1B@gmail.com>

src/backend/commands/proclang.c

@@ -463,7 +463,7 @@ find_language_template(const char *languageName)
 	ScanKeyInit(&key,
 				Anum_pg_pltemplate_tmplname,
 				BTEqualStrategyNumber, F_NAMEEQ,
-				NameGetDatum(languageName));
+				CStringGetDatum(languageName));
 	scan = systable_beginscan(rel, PLTemplateNameIndexId, true,
 							  NULL, 1, &key);