pgsql: Support partition pruning on boolcol IS [NOT] iso-8859-1

Support partition pruning on boolcol IS [NOT] UNKNOWN

While working on 4c2369ac5, I noticed we went out of our way not to
support clauses on boolean partitioned tables in the form of "IS
UNKNOWN" and "IS NOT UNKNOWN".  It's almost as much code to disallow
this as it is to allow it, so let's allow it.

Discussion: https://postgr.es/m/CAApHDvobKtcN6+xOuOfcutfp6T7jP=JPA9y3=MAEqnuKdDsQrw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/07c36c1333e00df5a4baf6b04ba2f3f300f57d69

Modified Files
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src/backend/partitioning/partprune.c          | 71 +++++++++++++++------------
src/test/regress/expected/partition_prune.out | 56 +++++++++++++++++----
src/test/regress/sql/partition_prune.sql      |  7 +++
3 files changed, 93 insertions(+), 41 deletions(-)