refactor: Split ATExecAlterConstraintInternal()

refactor: Split ATExecAlterConstraintInternal()

Split ATExecAlterConstraintInternal() into two functions:
ATExecAlterConstrDeferrability() and
ATExecAlterConstrInheritability().  This simplifies the code and
avoids unnecessary confusion caused by recursive code, which isn't
needed for ATExecAlterConstrInheritability().

(This also takes over the changes in commit 64224a834ce, as the new
AlterConstrDeferrabilityRecurse() is essentially the old
ATExecAlterChildConstr().)

Author: Amul Sul <amul.sul@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/CAAJ_b962c5AcYW9KUt_R_ER5qs3fUGbe4az-SP-vuwPS-w-AGA@mail.gmail.com