Allow partitionwise join to handle nested FULL JOIN USING cases.

This case didn't work because columns merged by FULL JOIN USING are
represented in the parse tree by COALESCE expressions, and the logic
for recognizing a partitionable join failed to match upper-level join
clauses to such expressions.  To fix, synthesize suitable COALESCE
expressions and add them to the nullable_partexprs lists.  This is
pretty ugly and brute-force, but it gets the job done.  (I have
ambitions of rethinking the way outer-join output Vars are
represented, so maybe that will provide a cleaner solution someday.
For now, do this.)

Amit Langote, reviewed by Justin Pryzby, Richard Guo, and myself

Discussion: https://postgr.es/m/CA+HiwqG2WVUGmLJqtR0tPFhniO=H=9qQ+Z3L_ZC+Y3-EVQHFGg@mail.gmail.com

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