[#1013] YSQL: Alter Column Type Part 2 - Re-arrange functions related…

… to cloning a table.

Summary:
It's purpose is to re-arrange code that is specifically related to cloning a table to the bottom of tablecmds.c to make it more readable. In particular we do the following:

- Move all YbAT* functions to the bottom of the file
- Rearrange the 4 YbAT* functions that are already at the bottom of the file to be in order of use

See [part 3](https://phabricator.dev.yugabyte.com/D24227) if more context is needed as to what changes are planned for altering the column type.

Test Plan:
The following tests involve adding or dropping primary keys:

1. TestPgAlterTableChangePrimaryKey
2. TestPgRegressTrigger
3. TestPgRegressPgStat
4. TestPgRegressIndex
5. TestPgRegressPgMiscIndependent
6. TestPgRegressWindow
7. TestPgRegressForeignData
8. TestPgRegressTable
9. TestPgRegressColocation

Their behavior must not change and their tests must pass.

Reviewers: dsrinivasan, tverona, jason, fizaa

Reviewed By: fizaa

Subscribers: yql

Differential Revision: https://phabricator.dev.yugabyte.com/D24224