[clang] Add AddClang.cmake to the list of the CMake modules that are installed

[awarzynski]

Summary

This makes sure that AddClang.cmake is installed alongside other Clang
CMake modules. This mirrors LLVM and MLIR in this respect and is
required when building the new Flang driver out of tree (as it depends
on Clang and includes AddClang.cmake).

EDIT
Some context info:

• Flang out-of-tree builds: https://github.com/llvm/llvm-project/tree/main/flang#building-flang-out-of-tree

Similar logic in:

• MLIR: https://github.com/llvm/llvm-project/blob/cbea6737d5130724c7c8cf8ee4ccf1c3dd099450/mlir/cmake/modules/CMakeLists.txt#L83

AddLLVM.cmake:

• LLVM: https://github.com/llvm/llvm-project/blob/cbea6737d5130724c7c8cf8ee4ccf1c3dd099450/llvm/cmake/modules/CMakeLists.txt#L150-L158

As this change mostly affects Flang, I'm adding some reviewers from that project too.

[awarzynski]

Ping.

Would anyone be able to take a look? Thank you!

[bogner]

This seems reasonable

clang/cmake/modules/CMakeLists.txt

if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
   get_property(clang_has_exports GLOBAL PROPERTY CLANG_HAS_EXPORTS)
   if(clang_has_exports)
     install(EXPORT ClangTargets DESTINATION ${CLANG_INSTALL_PACKAGE_DIR}
             COMPONENT clang-cmake-exports)
   endif()
 
   install(FILES
     ${CMAKE_CURRENT_BINARY_DIR}/CMakeFiles/ClangConfig.cmake
+    ${CMAKE_CURRENT_SOURCE_DIR}/AddClang.cmake
     DESTINATION ${CLANG_INSTALL_PACKAGE_DIR}
     COMPONENT clang-cmake-exports)
 
   if(NOT LLVM_ENABLE_IDE)
     # Add a dummy target so this can be used with LLVM_DISTRIBUTION_COMPONENTS
     add_custom_target(clang-cmake-exports)
     add_llvm_install_targets(install-clang-cmake-exports
                              COMPONENT clang-cmake-exports)
   endif()
 endif()