[clang] [clang] Implement dump() for MemberPointer APValues (PR #136130)

[Timm Baeder via cfe-commits]

tbaederr wrote:

> Can you do the JSONNodeDumper as well so the two stay somewhat in-sync?

Looks like that goes through `APValue::printPretty()`, so nothing to do for that.

https://github.com/llvm/llvm-project/pull/136130