[clang] Change ToLiteralEncodingConverter to a unique_ptr (PR #207258)

[Abhina Sree via cfe-commits]

abhina-sree wrote:

> @abhina-sree, can you explain what the cause of the failures were?

I not sure if I can reproduce this failure, and I didn't know which virtual function was being called. This patch was just a guess that the Converter was not being copied/destroyed properly.

https://github.com/llvm/llvm-project/pull/207258