[clang] [llvm] [HLSL] implementation of lerp intrinsic (PR #83077)

[Farzon Lotfi via cfe-commits]

farzonl wrote:

@python3kgae  I wanted to hold off on merging this one because I was investigating why [56240f0](https://github.com/llvm/llvm-project/commit/56240f07c39deb4c48bf7370dc72b3c67299eee1)   wiped out my changes on [d4384c5](https://github.com/llvm/llvm-project/commit/d4384c5ac52596eeab512574111e499f230b7db7). In any case I put fixes up on the frac intrinsic pr.

https://github.com/llvm/llvm-project/pull/83077