[llvm] [Offload][WIP] Move `/openmp/libomptarget` to `/offload` (PR #75125)

[Jan Patrick Lehr via llvm-commits]

jplehr wrote:

I gave this a shot with our buildbot CMake invocation to see what would happen and what we need to change, once this lands.
The CMake line the bot uses was modified to add the `offload` runtime, all else is the same.
```
cmake -G Ninja ../llvm -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=ON '-DLLVM_LIT_ARGS=-vv --show-unsupported --show-xfail -j 64' -DCMAKE_INSTALL_PREFIX=/tmp/llvm.inst -DCMAKE_BUILD_TYPE=Release '-DLLVM_TARGETS_TO_BUILD=X86;AMDGPU' '-DLLVM_ENABLE_RUNTIMES=offload;openmp' '-DLLVM_ENABLE_PROJECTS=clang;lld;llvm;compiler-rt'
```

I get errors about not finding something
```
CMake Error at $HOME/git/llvm-project/offload/CMakeLists.txt:126 (include):
  include could not find requested file:

    OpenMPTesting
```

I also get the following that I'm not sure what to do about (I don't get this in our regular buildbot case)
```
CMake Warning at $HOME/git/llvm-project/offload/CMakeLists.txt:182 (message):
  LTO is not supported: Change Dir:
  $HOME/git/llvm-project/botbuild/runtimes/runtimes-bins/offload/CMakeFiles/_CMakeLTOTest-CXX/bin

  Run Build Command(s):/usr/bin/ninja && [1/4] Building CXX object
  CMakeFiles/foo.dir/foo.cpp.o

  [2/4] Building CXX object CMakeFiles/boo.dir/main.cpp.o

  [3/4] Linking CXX static library libfoo.a

  [4/4] Linking CXX executable boo

  FAILED: boo

  : && $HOME/git/llvm-project/botbuild/./bin/clang++ -flto=thin
  CMakeFiles/boo.dir/main.cpp.o -o boo libfoo.a && :

  /usr/bin/ld:
  $HOME/git/llvm-project/botbuild/bin/../lib/LLVMgold.so: error
  loading plugin:
  $HOME/git/llvm-project/botbuild/bin/../lib/LLVMgold.so: cannot
  open shared object file: No such file or directory

  clang++: error: linker command failed with exit code 1 (use -v to see
  invocation)

  ninja: build stopped: subcommand failed.
```

https://github.com/llvm/llvm-project/pull/75125