[OpenMP] Port the OpenMP device runtime to direct C++ compilation (#123673)

Summary:
This removes the use of OpenMP offloading to build the device runtime.
The main benefit here is that we no longer need to rely on offloading
semantics to build a device only runtime. Things like variants are now
no longer needed and can just be simple if-defs. In the future, I will
remove most of the special handling here and fold it into calls to the
`<gpuintrin.h>` functions instead. Additionally I will rework the
compilation to make this a separate runtime.

The current plan is to have this, but make including OpenMP and
offloading either automatically add it, or print a warning if it's
missing. This will allow us to use a normal CMake workflow and delete
all the weird 'lets pull the clang binary out of the build' business.
```
-DRUNTIMES_amdgcn-amd-amdhsa_LLVM_ENABLE_RUNTIMES=offload
-DLLVM_RUNTIME_TARGETS=amdgcn-amd-amdhsa
```

After that, linking the OpenMP device runtime will be `-Xoffload-linker
-lomp`. I.e. no more fat binary business.

Only look at the most recent commit since this includes the two
dependencies
(fix to AMDGPUEmitPrintfBinding and the PointerToMember bug).

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