[clang] [clang] Add test for CWG156 "Name lookup for conversion functions" (PR #121654)

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cor3ntin wrote:

> To do that, we need to downgrade CWG1111 from being available in Clang 3.2 to "partial", and merge in at least the test that doesn't pass. Maybe we should do that anyway.

I'd say so, yes

https://github.com/llvm/llvm-project/pull/121654