Summary:
The test failed because it no longer passed Rpass by default without
LTO. I think that's desirable as it matches the standard behavior.
This reverts commit 6fd99de31864a5ef84ae8613b3a9034e05293461.

Summary;
Now that we use the linker to do LTO / device linking, we need to inform
the `clang` invocation to use `-flto` so it forwards arguments like
`-On` correctly.

omp_dynamic_shared_memory_mixed_amdgpu.c
omp_dynamic_shared_memory_amdgpu.c
amdgcn-amd-amdhsa::bug51982.c
amdgcn-amd-amdhsa::bug51781.c

In a nutshell, this moves our libomptarget code to populate the offload
subproject.

With this commit, users need to enable the new LLVM/Offload subproject
as a runtime in their cmake configuration.
No further changes are expected for downstream code.

Tests and other components still depend on OpenMP and have also not been
renamed. The results below are for a build in which OpenMP and Offload
are enabled runtimes. In addition to the pure `git mv`, we needed to
adjust some CMake files. Nothing is intended to change semantics.

```
ninja check-offload
```
Works with the X86 and AMDGPU offload tests

```
ninja check-openmp
```
Still works but doesn't build offload tests anymore.

```
ls install/lib
```
Shows all expected libraries, incl.
- `libomptarget.devicertl.a`
- `libomptarget-nvptx-sm_90.bc`
- `libomptarget.rtl.amdgpu.so` -> `libomptarget.rtl.amdgpu.so.18git`
- `libomptarget.so` -> `libomptarget.so.18git`

Fixes: https://github.com/llvm/llvm-project/issues/75124

---------

Co-authored-by: Saiyedul Islam <Saiyedul.Islam@amd.com>