[clang] [HLSL] Implement the `lit` intrinsic (PR #134171)

[Chris B via cfe-commits]

llvm-beanz wrote:

> Given your feedback here it sounds like you want us to drop the template and allow for implicit vector truncation so that the error will be ambiguous instead of `call to deleted function 'lit'`?

Yes, let's not add compatibility overloads unless we have driving reasons for them.

https://github.com/llvm/llvm-project/pull/134171