pgsql: Remove bogus Assert, add some regression test cases showing why.

Remove bogus Assert, add some regression test cases showing why.

Commit 77ec5affb added an assertion to enforce_generic_type_consistency
that boils down to "if the function result is polymorphic, there must be
at least one polymorphic argument". This should be true for user-created
functions, but there are built-in functions for which it's not true, as
pointed out by Jaime Casanova. Hence, go back to the old behavior of
leaving the return type alone. There's only a limited amount of stuff
you can do with such a function result, but it does work to some extent;
add some regression test cases to ensure we don't break that again.

Discussion: https://postgr.es/m/CAJGNTeMbhtsCUZgJJ8h8XxAJbK7U2ipsX8wkHRtZRz-NieT8RA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/07871d40c72e498b6e034eb674df5d8d206976bc

Modified Files
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src/backend/parser/parse_coerce.c | 21 ++++++++++++---------
src/test/regress/expected/polymorphism.out | 13 +++++++++++++
src/test/regress/sql/polymorphism.sql | 5 +++++
3 files changed, 30 insertions(+), 9 deletions(-)