The ALTER TABLE .. COALESCE PARTITION feature is while partially
implemented not supported. Removing all the scaffolding around the
parsing might as well be worthwhile but at least it seems reasonable
to kill the completely dead code in ATPExecPartCoalesce(). As this
was the only external caller of parruleord_open_gap() make the
function static.

atpxPartAddList() needs a CreateStmt that represents the parent table,
but instead of creating it already in the parser, and adding more details
to it in analyze.c, it's simpler to create it later, in atpxPartAddList(),
where it's actually needed.

This saves a little bit of memory when parsing massively partitioned
CREATE TABLE statements.

We always passed CurrentMemoryContext for them, so might as well remove
the parameter, making the code more readable, and always allocate the
return values in CurrentMemoryContext.

parser/analyze.c is bloated, moving larger chunks of GPDB-specific
functionality like this elsewhere makes it more readable. Make diffing and
merging of analyze.c with upstream easier, too.

I also ran pgindent on parse_partition.c.

These were both only used by a single, static, function.