move PartitionBoundInfo creation code

Hi,

Currently, the code that creates a PartitionBoundInfo struct from the
PartitionBoundSpec nodes of constituent partitions read from the catalog
is in RelationBuildPartitionDesc that's in partcache.c. I think that
da6f3e45dd that moved around the partitioning code [1] really missed the
opportunity to move the aforementioned code into partbounds.c. I think
there is quite a bit of logic contained in that code that can aptly be
said to belong in partbounds.c. Also, if factored out into a function of
its own with a proper interface, it could be useful to other callers that
may want to build it for, say, fake partitions [2] which are not real
relations.

Attached find a patch that does such refactoring, along with making some
functions in partbounds.c that are not needed outside static.

Thanks,
Amit

[1] https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=da6f3e45ddb

[2]
https://www.postgresql.org/message-id/009901d3f34c%2471e1bdc0%2455a53940%24%40lab.ntt.co.jp