[HLSL] implement exp intrinsic (#83832)

This change implements: #70072

- `hlsl_intrinsics.h` - add the `exp` api
- `DXIL.td` - add the llvm intrinsic to DXIL opcode lowering mapping.
- This change reuses llvm's existing intrinsic
`__builtin_elementwise_exp` \ `int_exp` & `__builtin_elementwise_exp2` \
`int_exp2`
- This PR is part 1 of 2.
- Part 2 requires an intrinsic to instructions lowering.
Part2 will expand `int_exp` to 
```
A = Builder.CreateFMul(log2eConst, val);
int_exp2(A)
```
just like we do in
[TranslateExp](https://github.com/microsoft/DirectXShaderCompiler/blob/main/lib/HLSL/HLOperationLower.cpp#L2220C1-L2236C2)