[SPIRV][HLSL] map lerp to Fmix (#88976)

- `clang/lib/CodeGen/CGBuiltin.cpp` - switch to using
`getLerpIntrinsic()` to abstract backend intrinsic
- `clang/lib/CodeGen/CGHLSLRuntime.h` - add `getLerpIntrinsic()` 
- `llvm/include/llvm/IR/IntrinsicsSPIRV.td` - add SPIRV intrinsic for
lerp
- `llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp` - add mapping of
HLSL's lerp to GLSL's Fmix.

resolves #88940