[clang] [Format] Configure ASSIGN_OR_RETURN macros for Google style (PR #169037)

[via cfe-commits]

https://github.com/owenca commented:

Please note that user-added `Macros` will overwrite the default:
```
$ clang-format -dump-config -style='{BasedOnStyle: Google, Macros: [A(x)=x]}' | grep -w -A2 Macros:
Macros:
  - 'A(x)=x'
  - 'ASSIGN_OR_RETURN(a, b, c)=a = (b); if (x) return c'
```

https://github.com/llvm/llvm-project/pull/169037