[clang] [HLSL] Make lerp overload tests stricter. NFC (PR #137898)

[Farzon Lotfi via cfe-commits]

farzonl wrote:

I have a strong suspicion  all the `*-overload.hlsl` tests  have the same mistake and are also incorrect and need to updated. Can you file a ticket for investigating\cleaning up the other overload tests. Not work you have to do, but we should at least track that it needs ot be done.

https://github.com/llvm/llvm-project/pull/137898