[clang] [llvm] [openmp] [OpenMP] Use generic IR for the OpenMP DeviceRTL (PR #119091)

[Shilei Tian via cfe-commits]

shiltian wrote:

> The IR is a representation of what goes on a final .o. It is not really intended to enable new features or make the program portable.

My .02 is, it depends on how to think about it. If we eliminate the "unwanted" part (especially those that could potentially affect ABI) at the very beginning of the middle end, it will not have optimization effects.

In addition, the compiler backend lowering is essentially doing something like:

```
if (target == A)
  lower Inst to I1
else if (has_feature_that_only_compiler_can_call("..."))
  lower Inst to I2
...
```

If speaking from "portability", the compiler is "portable". The reflection is just to give end user an interface to do similar things:

```
if (__builtin_get_target() == A)
  do something in one way
else if (has_feature_that_user_can_call("..."))
  do something in another way
...
```

Of course the interface(s) have to be carefully designed and all the queried information can't be changed during optimization.

https://github.com/llvm/llvm-project/pull/119091