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