Allow most keywords to be used as column labels without requiring AS.

Up to now, if you tried to omit "AS" before a column label in a SELECT
list, it would only work if the column label was an IDENT, that is not
any known keyword.  This is rather unfriendly considering that we have
so many keywords and are constantly growing more.  In the wake of commit
1ed6b8956 it's possible to improve matters quite a bit.

We'd originally tried to make this work by having some of the existing
keyword categories be allowed without AS, but that didn't work too well,
because each category contains a few special cases that don't work
without AS.  Instead, invent an entirely orthogonal keyword property
"can be bare column label", and mark all keywords that way for which
we don't get shift/reduce errors by doing so.

It turns out that of our 450 current keywords, all but 39 can be made
bare column labels, improving the situation by over 90%.  This number
might move around a little depending on future grammar work, but it's
a pretty nice improvement.

Mark Dilger, based on work by myself and Robert Haas;
review by John Naylor

Discussion: https://postgr.es/m/38ca86db-42ab-9b48-2902-337a0d6b8311@2ndquadrant.com

src/common/keywords.c

@@ -24,10 +24,25 @@
 
 /* Keyword categories for SQL keywords */
 
-#define PG_KEYWORD(kwname, value, category) category,
+#define PG_KEYWORD(kwname, value, category, collabel) category,
 
 const uint8 ScanKeywordCategories[SCANKEYWORDS_NUM_KEYWORDS] = {
 #include "parser/kwlist.h"
 };
 
 #undef PG_KEYWORD
+
+/* Keyword can-be-bare-label flags for SQL keywords */
+
+#define PG_KEYWORD(kwname, value, category, collabel) collabel,
+
+#define BARE_LABEL true
+#define AS_LABEL false
+
+const bool	ScanKeywordBareLabel[SCANKEYWORDS_NUM_KEYWORDS] = {
+#include "parser/kwlist.h"
+};
+
+#undef PG_KEYWORD
+#undef BARE_LABEL
+#undef AS_LABEL