llvmir2hll: Treat LLVM Intrinsic functions as Instruction-Idiom Functions

Functions tagged as idiom functions in our config get the following comment in the output C (example for `fabsf`):
```cpp
// --------------- Instruction-Idiom Functions ----------------

// float32_t fabsf(float32_t a1);
```
It is often easier and cleaner to generate LLVM Intrinsic function, instead of an ordinary function that gets tagged with idiom flag - e.g. generate `@llvm.fabs.f32` without a flag, instead of `fabsf` with flag.
These LLVM Intrinsics are handled by llvmir2hll, but comments like the one above are not generated for the resulting functions. E.g. calls of `@llvm.fabs.f32` are generated to C as `fabsf`, but without the comment above.

Add the comment for all functions that are created from LLVM Intrinsics, regardless the tag in our config.

This is tested in `features.func-separation.IdiomFromFrontendTest`, which I disabled until this is fixed - `@llvm.fabs.f32` is generated in bin2llvmir instead of `fabsf` and the tested comment is no longer in the output C.