[NFC] [Serialization] Turn type alias LocalDeclID into class
Previously, the LocalDeclID and GlobalDeclID are defined as:
```
using LocalDeclID = DeclID;
using GlobalDeclID = DeclID;
```
This is more or less concerning that we may misuse LocalDeclID and
GlobalDeclID without understanding it. There is also a FIXME saying
this.
This patch tries to turn LocalDeclID into a class to improve the type
safety here.
llvm-project
b8e3b2ad - [NFC] [Serialization] Turn type alias LocalDeclID into class
