[CMake] Enable buildings builtins for Darwin as part of runtimes

[phosek]

Summary

This seems to be working fine in my testing so this error check shouldn't be needed.

[beanz]

I'm suspicious that this would work as intended. What builtin libraries get built if your triple is Darwin?

[beanz]

So... Looking at the CMake in compiler-rt, COMPILER_RT_DEFAULT_TARGET_ONLY is only interpreted in a NOT APPLE block, so this would really not behave the way you intend. I think it is better to not support this on Darwin for the time being and provide a hard error rather than behaving incorrectly.

I suspect if you passed in multiple Darwin triples in LLVM_BUILTIN_TARGETS you'd get multiple copies of all the Darwin builtin libraries being built.

[phosek]

In D35346#813474, @beanz wrote:

So... Looking at the CMake in compiler-rt, COMPILER_RT_DEFAULT_TARGET_ONLY is only interpreted in a NOT APPLE block, so this would really not behave the way you intend. I think it is better to not support this on Darwin for the time being and provide a hard error rather than behaving incorrectly.

I suspect if you passed in multiple Darwin triples in LLVM_BUILTIN_TARGETS you'd get multiple copies of all the Darwin builtin libraries being built.

Would it be OK to avoid throwing error in case the $target is default (even on Darwin)? That's all I need at the moment.

runtimes/CMakeLists.txt

else()
       get_cmake_property(variableNames VARIABLES)
       add_custom_target(builtins)
       add_custom_target(install-builtins)
       foreach(target ${LLVM_BUILTIN_TARGETS})
         if(target STREQUAL "default")
           set(target ${LLVM_DEFAULT_TARGET_TRIPLE})
         endif()
 
-        string(REPLACE "-" ";" builtin_target_list ${target})
-        foreach(item ${builtin_target_list})
-          string(TOLOWER "${item}" item_lower)
-          if(item_lower MATCHES "darwin")
-            message(FATAL_ERROR "LLVM_BUILTIN_TARGETS isn't implemented for Darwin platform!")
-          endif()
-        endforeach()
-
         foreach(variableName ${variableNames})
           if(variableName MATCHES "^BUILTINS_${target}")
             string(REPLACE "BUILTINS_${target}_" "" new_name ${variableName})
             list(APPEND ${target}_extra_args "-D${new_name}=${${variableName}}")
           endif()
         endforeach()
         llvm_ExternalProject_Add(builtins-${target}
                                ${CMAKE_CURRENT_SOURCE_DIR}/compiler-rt/lib/builtins