Fix handling of partition bounds for boolean partitioning columns.

Fix handling of partition bounds for boolean partitioning columns.

Previously, you could partition by a boolean column as long as you
spelled the bound values as string literals, for instance FOR VALUES
IN ('t').  The trouble with this is that ruleutils.c printed that as
FOR VALUES IN (TRUE), which is reasonable syntax but wasn't accepted by
the grammar.  That results in dump-and-reload failures for such cases.

Apply a minimal fix that just causes TRUE and FALSE to be converted to
strings 'true' and 'false'.  This is pretty grotty, but it's too late for
a more principled fix in v11 (to say nothing of v10).  We should revisit
the whole issue of how partition bound values are parsed for v12.

Amit Langote

Discussion: https://postgr.es/m/e05c5162-1103-7e37-d1ab-6de3e0afaf70@lab.ntt.co.jp