Support plpgsql variable names that conflict with unreserved SQL keywords.

A variable name matching a statement-introducing keyword, such as
"comment" or "update", caused parse failures if one tried to write
a statement using that keyword.  Commit bb1b8f69 already addressed
this scenario for the case of variable names matching unreserved
plpgsql keywords, but we didn't think about unreserved core-grammar
keywords.  The same heuristic (viz, it can't be a variable name
unless the next token is assignment or '[') should work fine for
that case too, and as a bonus the code gets shorter and less
duplicative.

Per bug #15555 from Feike Steenbergen.  Since this hasn't been
complained of before, and is easily worked around anyway,
I won't risk a back-patch.

Discussion: https://postgr.es/m/15555-149bbd70ddc7b4b6@postgresql.org

src/test/regress/expected/plpgsql.out

@@ -4781,6 +4781,27 @@ select unreserved_test();
               43
 (1 row)
 
+create or replace function unreserved_test() returns int as $$
+declare
+  comment int := 21;
+begin
+  comment := comment * 2;
+  comment on function unreserved_test() is 'this is a test';
+  return comment;
+end
+$$ language plpgsql;
+select unreserved_test();
+ unreserved_test 
+-----------------
+              42
+(1 row)
+
+select obj_description('unreserved_test()'::regprocedure, 'pg_proc');
+ obj_description 
+-----------------
+ this is a test
+(1 row)
+
 drop function unreserved_test();
 --
 -- Test FOREACH over arrays

src/test/regress/sql/plpgsql.sql

@@ -3892,6 +3892,20 @@ $$ language plpgsql;
 
 select unreserved_test();
 
+create or replace function unreserved_test() returns int as $$
+declare
+  comment int := 21;
+begin
+  comment := comment * 2;
+  comment on function unreserved_test() is 'this is a test';
+  return comment;
+end
+$$ language plpgsql;
+
+select unreserved_test();
+
+select obj_description('unreserved_test()'::regprocedure, 'pg_proc');
+
 drop function unreserved_test();
 
 --