[Offload] Provide a CMake cache file to easily build offloading (#115074)

Summary:
This patch adds a cache file that will automatically enable openpm,
offload, and all the fancy GPU libraries.

Author: jhuber6

offload/cmake/caches/Offload.cmake

@@ -0,0 +1,12 @@
+set(LLVM_ENABLE_PROJECTS "clang;clang-tools-extra;compiler-rt;lld" CACHE STRING "")
+set(LLVM_ENABLE_RUNTIMES "libunwind;libcxx;libcxxabi;openmp;offload" CACHE STRING "")
+set(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR ON CACHE BOOL "")
+
+set(CLANG_DEFAULT_CXX_STDLIB "libc++" CACHE STRING "")
+set(CLANG_DEFAULT_LINKER "lld" CACHE STRING "")
+
+set(LLVM_RUNTIME_TARGETS default;amdgcn-amd-amdhsa;nvptx64-nvidia-cuda CACHE STRING "") 
+set(RUNTIMES_nvptx64-nvidia-cuda_CACHE_FILES "${CMAKE_SOURCE_DIR}/../libcxx/cmake/caches/NVPTX.cmake" CACHE STRING "")
+set(RUNTIMES_amdgcn-amd-amdhsa_CACHE_FILES "${CMAKE_SOURCE_DIR}/../libcxx/cmake/caches/AMDGPU.cmake" CACHE STRING "")
+set(RUNTIMES_nvptx64-nvidia-cuda_LLVM_ENABLE_RUNTIMES "compiler-rt;libc;libcxx;libcxxabi" CACHE STRING "")
+set(RUNTIMES_amdgcn-amd-amdhsa_LLVM_ENABLE_RUNTIMES "compiler-rt;libc;libcxx;libcxxabi" CACHE STRING "")

openmp/docs/SupportAndFAQ.rst

@@ -51,7 +51,23 @@ All patches go through the regular `LLVM review process
 
 Q: How to build an OpenMP GPU offload capable compiler?
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-To build an *effective* OpenMP offload capable compiler, only one extra CMake
+
+The easiest way to create an offload capable compiler is to use the provided 
+CMake cache file. This will enable the projects and runtimes necessary for 
+offloading as well as some extra options.
+
+.. code-block:: sh
+
+  $> cd llvm-project  # The llvm-project checkout
+  $> mkdir build
+  $> cd build
+  $> cmake ../llvm -G Ninja                                                 \
+     -C ../offload/cmake/caches/Offload.cmake \ # The preset cache file
+     -DCMAKE_BUILD_TYPE=<Debug|Release>   \ # Select build type
+     -DCMAKE_INSTALL_PREFIX=<PATH>        \ # Where the libraries will live
+  $> ninja install
+
+To manually build an *effective* OpenMP offload capable compiler, only one extra CMake
 option, ``LLVM_ENABLE_RUNTIMES="openmp;offload"``, is needed when building LLVM (Generic
 information about building LLVM is available `here
 <https://llvm.org/docs/GettingStarted.html>`__.). Make sure all backends that