[Offload][CUDA] Allow CUDA kernels to use LLVM/Offload (#94549)

Through the new `-foffload-via-llvm` flag, CUDA kernels can now be
lowered to the LLVM/Offload API. On the Clang side, this is simply done
by using the OpenMP offload toolchain and emitting calls to `llvm*`
functions to orchestrate the kernel launch rather than `cuda*`
functions. These `llvm*` functions are implemented on top of the
existing LLVM/Offload API.

As we are about to redefine the Offload API, this wil help us in the
design process as a second offload language.

We do not support any CUDA APIs yet, however, we could:
  https://www.osti.gov/servlets/purl/1892137

For proper host execution we need to resurrect/rebase
  https://tianshilei.me/wp-content/uploads/2021/12/llpp-2021.pdf
(which was designed for debugging).

```
❯❯❯ cat test.cu
extern "C" {
void *llvm_omp_target_alloc_shared(size_t Size, int DeviceNum);
void llvm_omp_target_free_shared(void *DevicePtr, int DeviceNum);
}

__global__ void square(int *A) { *A = 42; }

int main(int argc, char **argv) {
  int DevNo = 0;
  int *Ptr = reinterpret_cast<int *>(llvm_omp_target_alloc_shared(4, DevNo));
  *Ptr = 7;
  printf("Ptr %p, *Ptr %i\n", Ptr, *Ptr);
  square<<<1, 1>>>(Ptr);
  printf("Ptr %p, *Ptr %i\n", Ptr, *Ptr);
  llvm_omp_target_free_shared(Ptr, DevNo);
}

❯❯❯ clang++ test.cu -O3 -o test123 -foffload-via-llvm --offload-arch=native

❯❯❯ llvm-objdump --offloading test123

test123:        file format elf64-x86-64

OFFLOADING IMAGE [0]:
kind            elf
arch            gfx90a
triple          amdgcn-amd-amdhsa
producer        openmp

❯❯❯ LIBOMPTARGET_INFO=16 ./test123
Ptr 0x155448ac8000, *Ptr 7
Ptr 0x155448ac8000, *Ptr 42
```

1 files changed, 2 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CodeGenFunction.cpp b/clang/lib/CodeGen/CodeGenFunction.cpp
index 2b2e23f..7adc5a4 100644
--- a/clang/lib/CodeGen/CodeGenFunction.cpp
+++ b/clang/lib/CodeGen/CodeGenFunction.cpp
@@ -1013,7 +1013,8 @@ void CodeGenFunction::StartFunction(GlobalDecl GD, QualType RetTy,
   }
 
   if (FD && (getLangOpts().OpenCL ||
-             (getLangOpts().HIP && getLangOpts().CUDAIsDevice))) {
+             ((getLangOpts().HIP || getLangOpts().OffloadViaLLVM) &&
+              getLangOpts().CUDAIsDevice))) {
     // Add metadata for a kernel function.
     EmitKernelMetadata(FD, Fn);
   }