Standardize the definition of the subtype field of AlterDomainStmt

I noticed that the subtype of AlterDomainStmt is directly using
constants in the code. It is not conducive to the maintenance and
reading of the code. Based on the definition of AlterTableType, use
"AD_" as the prefix. Define several macros to replace the original
characters.
The subtype of AlterTableCmd is defined using an enumeration. The
subtypes of AlterDomainStmt are relatively few in number, and the
original definition uses characters. These definitions still use
characters and maintain the values unchanged. If some plugins or tools
are also processing AlterDomainStmt, there will be no errors.

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Quan Zongliang