[clang] [clang][NFC] Extract LookupOverloadedUnaryOp (PR #210567)

[Matthias Wippich via cfe-commits]

Tsche wrote:

@Sirraide pinging you since we talked about this on discord (in the context of better diagnostics for `[[clang::flag_enum]]`).

https://github.com/llvm/llvm-project/pull/210567