sql: compat: Support 'COMMENT ON TABLE foo is 'bar' parsing

Many ORMs use Postgres' COMMENT statement to label database objects. This commit
handles the grammar changes necessary to support the COMMENT syntax for
Columns and Tables. The implementaiton of COMMENT is left for a future PR.

Release note (sql change): allow parsing of COMMENT ON syntax

docs/codelabs/01-sql-statement.md

@@ -295,7 +295,7 @@ a method to the planner.  That's where the centralized statement dispatch takes
 place, so that's the place to add semantics.
 
 Look for the source of the error we're seeing.  You'll find that it's at the end
-of a long type switch statement.  Let's add a case to that:
+of a long type switch statement in `/pkg/sql/plan.go`.  Let's add a case to that:
 
 ```go
 case *tree.Frobnicate:

pkg/sql/parser/sql.y

@@ -435,7 +435,7 @@ func (u *sqlSymUnion) scrubOption() tree.ScrubOption {
 
 %token <str>   CACHE CANCEL CASCADE CASE CAST CHAR
 %token <str>   CHARACTER CHARACTERISTICS CHECK
-%token <str>   CLUSTER COALESCE COLLATE COLLATION COLUMN COLUMNS COMMIT
+%token <str>   CLUSTER COALESCE COLLATE COLLATION COLUMN COLUMNS COMMENT COMMIT
 %token <str>   COMMITTED COMPACT CONCAT CONFIGURATION CONFIGURATIONS CONFIGURE
 %token <str>   CONFLICT CONSTRAINT CONSTRAINTS CONTAINS COPY COVERING CREATE
 %token <str>   CROSS CSV CUBE CURRENT CURRENT_CATALOG CURRENT_DATE CURRENT_SCHEMA
@@ -590,6 +590,7 @@ func (u *sqlSymUnion) scrubOption() tree.ScrubOption {
 %type <tree.ScrubOptions> scrub_option_list
 %type <tree.ScrubOption> scrub_option
 
+%type <tree.Statement> comment_stmt
 %type <tree.Statement> commit_stmt
 %type <tree.Statement> copy_from_stmt
 
@@ -671,6 +672,7 @@ func (u *sqlSymUnion) scrubOption() tree.ScrubOption {
 %type <tree.Statement> show_zone_stmt
 
 %type <str> session_var
+%type <str> comment_text
 
 %type <tree.Statement> transaction_stmt
 %type <tree.Statement> truncate_stmt
@@ -994,6 +996,7 @@ stmt:
 | cancel_stmt     // help texts in sub-rule
 | scrub_stmt
 | copy_from_stmt
+| comment_stmt    // EXTEND WITH HELP: COMMENT
 | create_stmt     // help texts in sub-rule
 | deallocate_stmt // EXTEND WITH HELP: DEALLOCATE
 | delete_stmt     // EXTEND WITH HELP: DELETE
@@ -1640,6 +1643,24 @@ cancel_query_stmt:
   }
 | CANCEL QUERY error // SHOW HELP: CANCEL QUERY
 
+// %Help: COMMENT - add a comment to a database object
+// %Category: Misc
+// %Text:
+// COMMENT ON [ COLUMN | TABLE ] <objname> IS { 'text' | NULL }
+comment_stmt:
+  COMMENT ON TABLE any_name IS comment_text
+  {
+    return unimplemented(sqllex, "COMMENT ON TABLE unimplemented")
+  }
+| COMMENT ON COLUMN any_name IS comment_text
+  {
+   return unimplemented(sqllex, "COMMENT ON COLUMN unimplemented")
+  }
+
+comment_text:
+			SCONST							{ $$ = $1 }
+			| NULL							{ $$ = "" }
+
 // %Help: CREATE
 // %Category: Group
 // %Text:
@@ -7068,6 +7089,7 @@ unreserved_keyword:
 | CASCADE
 | CLUSTER
 | COLUMNS
+| COMMENT
 | COMMIT
 | COMMITTED
 | COMPACT