Improve makeArrayTypeName's algorithm for choosing array type names.

[lss602726449]

As before, we start by prepending one underscore (truncating the base name if necessary). But if there is a conflict, then instead of prepending more and more underscores, append an underscore and some digits, in much the same way that ChooseRelationName does. While the previous logic could be driven to fail by creating a lot of types with long names differing only near the end, this version seems certain enough to eventually succeed that we can remove the failure code path that was there before.

While at it, undo 6df7a96's decision to split this code out of makeArrayTypeName. That wasn't actually accomplishing anything, because no other function was using it --- and it would have been wrong to do so. The convention that a prefix "_" means an array, not something else, is too ancient to mess with.

Andrey Lepikhov and Dmitry Koval, reviewed by Masahiko Sawada and myself

Discussion: https://postgr.es/m/[email protected]

closes: #ISSUE_Number

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